diff --git a/activitystreams-derive/src/lib.rs b/activitystreams-derive/src/lib.rs
index d60f5b9..7ecf797 100644
--- a/activitystreams-derive/src/lib.rs
+++ b/activitystreams-derive/src/lib.rs
@@ -52,7 +52,7 @@ pub fn unit_string(input: TokenStream) -> TokenStream {
         .unwrap()
         .clone();
 
-    let value = get_value(attr);
+    let value = from_value(attr);
 
     let visitor_name = Ident::from(format!("{}Visitor", value));
 
@@ -118,7 +118,7 @@ pub fn unit_string(input: TokenStream) -> TokenStream {
     c.into()
 }
 
-fn get_value(attr: Attribute) -> String {
+fn from_value(attr: Attribute) -> String {
     let group = attr.tts
         .clone()
         .into_iter()
@@ -218,12 +218,19 @@ pub fn properties_derive(input: TokenStream) -> TokenStream {
                     let lower_variant = variant.to_lowercase();
                     let fn_name = Ident::from(format!("{}_{}", ident, lower_variant));
                     let fn_plural = Ident::from(format!("{}_{}_vec", ident, lower_variant));
+                    let set_fn_name = Ident::from(format!("set_{}_{}", ident, lower_variant));
+                    let set_fn_plural = Ident::from(format!("set_{}_{}_vec", ident, lower_variant));
                     let variant = Ident::from(variant);
 
                     if is_concrete && is_option {
                         let single = quote! {
                             pub fn #fn_name(&self) -> ::error::Result<#variant> {
-                                ::properties::get_item(&self.#ident)
+                                ::properties::from_item(&self.#ident)
+                            }
+
+                            pub fn #set_fn_name(&mut self, item: #variant) -> ::error::Result<()> {
+                                self.#ident = ::properties::to_item(item)?;
+                                Ok(())
                             }
                         };
 
@@ -234,20 +241,35 @@ pub fn properties_derive(input: TokenStream) -> TokenStream {
                                 #single
 
                                 pub fn #fn_plural(&self) -> ::error::Result<Vec<#variant>> {
-                                    ::properties::get_item(&self.#ident)
+                                    ::properties::from_item(&self.#ident)
+                                }
+
+                                pub fn #set_fn_plural(&mut self, item: Vec<#variant>) -> ::error::Result<()> {
+                                    self.#ident = ::properties::to_item(item)?;
+                                    Ok(())
                                 }
                             }
                         }
                     } else if is_concrete && is_vec {
                         quote! {
                             pub fn #fn_name(&self) -> ::error::Result<Vec<#variant>> {
-                                ::properties::get_vec(&self.#ident)
+                                ::properties::from_vec(&self.#ident)
+                            }
+
+                            pub fn #set_fn_name(&mut self, item: Vec<#variant>>) -> ::error::Result<()> {
+                                self.#ident = ::properties::to_vec(item)?;
+                                Ok(())
                             }
                         }
                     } else if is_concrete {
                         let single = quote! {
                             pub fn #fn_name(&self) -> ::error::Result<#variant> {
-                                ::properties::get_value(&self.#ident)
+                                ::properties::from_value(&self.#ident)
+                            }
+
+                            pub fn #set_fn_name(&mut self, item: #variant) -> ::error::Result<()> {
+                                self.#ident = ::properties::to_value(item)?;
+                                Ok(())
                             }
                         };
 
@@ -258,14 +280,24 @@ pub fn properties_derive(input: TokenStream) -> TokenStream {
                                 #single
 
                                 pub fn #fn_plural(&self) -> ::error::Result<Vec<#variant>> {
-                                    ::properties::get_value(&self.#ident)
+                                    ::properties::from_value(&self.#ident)
+                                }
+
+                                pub #set_fn_plural(&mut self, item: Vec<#variant>) -> ::error::Result<()> {
+                                    self.#ident = ::properties::to_value(item)?;
+                                    Ok(())
                                 }
                             }
                         }
                     } else if is_option {
                         let single = quote! {
                             pub fn #fn_name<T: #variant>(&self) -> ::error::Result<T> {
-                                ::properties::get_item(&self.#ident)
+                                ::properties::from_item(&self.#ident)
+                            }
+
+                            pub fn #set_fn_name<T: #variant>(&mut self, item: T) -> ::error::Result<()> {
+                                self.#ident = ::properties::to_item(item)?;
+                                Ok(())
                             }
                         };
 
@@ -276,20 +308,35 @@ pub fn properties_derive(input: TokenStream) -> TokenStream {
                                 #single
 
                                 pub fn #fn_plural<T: #variant>(&self) -> ::error::Result<Vec<T>> {
-                                    ::properties::get_item(&self.#ident)
+                                    ::properties::from_item(&self.#ident)
+                                }
+
+                                pub fn #set_fn_plural<T: #variant>(&mut self, item: Vec<T>) -> ::error::Result<()> {
+                                    self.#ident = ::properties::to_item(item)?;
+                                    Ok(())
                                 }
                             }
                         }
                     } else if is_vec {
                         quote! {
                             pub fn #fn_name<T: #variant>(&self) -> ::error::Result<Vec<T>> {
-                                ::properties::get_vec(&self.#ident)
+                                ::properties::from_vec(&self.#ident)
+                            }
+
+                            pub fn #set_fn_name<T: #variant>(&mut self, item: Vec<T>) -> ::error::Result<()> {
+                                self.#ident = ::properties::to_vec(item)?;
+                                Ok(())
                             }
                         }
                     } else {
                         let single = quote! {
                             pub fn #fn_name<T: #variant>(&self) -> ::error::Result<T> {
-                                ::properties::get_value(&self.#ident)
+                                ::properties::from_value(&self.#ident)
+                            }
+
+                            pub fn #set_fn_name<T: #variant>(&mut self, item: T) -> ::error::Result<()> {
+                                self.#ident = ::properties::to_value(item)?;
+                                Ok(())
                             }
                         };
 
@@ -300,7 +347,12 @@ pub fn properties_derive(input: TokenStream) -> TokenStream {
                                 #single
 
                                 pub fn #fn_plural<T: #variant>(&self) -> ::error::Result<Vec<T>> {
-                                    ::properties::get_value(&self.#ident)
+                                    ::properties::from_value(&self.#ident)
+                                }
+
+                                pub fn #set_fn_plural<T: #variant>(&mut self, item: Vec<T>) -> ::error::Result<()> {
+                                    self.#ident = ::properties::to_value(item)?;
+                                    Ok(())
                                 }
                             }
                         }
diff --git a/src/custom_props.rs b/src/custom_props.rs
new file mode 100644
index 0000000..752ccc1
--- /dev/null
+++ b/src/custom_props.rs
@@ -0,0 +1,56 @@
+/*
+ * This file is part of ActivityStreams.
+ *
+ * Copyright © 2018 Riley Trautman
+ *
+ * ActivityStreams is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * ActivityStreams is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with ActivityStreams.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+use link::Link;
+use object::Object;
+
+#[derive(Clone, Debug, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct CustomLink<C, L> {
+    #[serde(flatten)]
+    link: L,
+
+    #[serde(flatten)]
+    pub custom_props: C,
+}
+
+impl<C, L: Link> CustomLink<C, L> {
+    pub fn new(link: L, custom_props: C) -> Self {
+        CustomLink { link, custom_props }
+    }
+}
+
+#[derive(Clone, Debug, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct CustomObject<C, O> {
+    #[serde(flatten)]
+    object: O,
+
+    #[serde(flatten)]
+    pub custom_props: C,
+}
+
+impl<C, O: Object> CustomObject<C, O> {
+    pub fn new(object: O, custom_props: C) -> Self {
+        CustomObject {
+            object,
+            custom_props,
+        }
+    }
+}
diff --git a/src/error.rs b/src/error.rs
index fb022b3..81998ed 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -23,8 +23,12 @@ use std::result;
 pub enum Error {
     #[fail(display = "Key not present")]
     NotFound,
+
     #[fail(display = "Failed to deserialize data as requested type")]
     Deserialize,
+
+    #[fail(display = "Failed to serialize data")]
+    Serialize,
 }
 
 pub type Result<T> = result::Result<T, Error>;
diff --git a/src/lib.rs b/src/lib.rs
index 6bb3f75..f039f87 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -38,6 +38,7 @@ pub fn context() -> serde_json::Value {
 pub mod activity;
 pub mod actor;
 pub mod collection;
+pub mod custom_props;
 pub mod error;
 pub mod link;
 pub mod object;
diff --git a/src/properties.rs b/src/properties.rs
index 3fabbb9..f7e6344 100644
--- a/src/properties.rs
+++ b/src/properties.rs
@@ -1,38 +1,81 @@
-use serde::de::DeserializeOwned;
+/*
+ * This file is part of ActivityStreams.
+ *
+ * Copyright © 2018 Riley Trautman
+ *
+ * ActivityStreams is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * ActivityStreams is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with ActivityStreams.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+use serde::{de::DeserializeOwned, ser::Serialize};
 use serde_json;
 
 use error::{Error, Result};
 
-pub fn get_value<I>(item: &serde_json::Value) -> Result<I>
+pub fn from_value<I>(item: &serde_json::Value) -> Result<I>
 where
     I: DeserializeOwned,
 {
     serde_json::from_value(item.clone()).map_err(|_| Error::Deserialize)
 }
 
-pub fn get_item<I>(item: &Option<serde_json::Value>) -> Result<I>
+pub fn to_value<I>(item: I) -> Result<serde_json::Value>
+where
+    I: Serialize,
+{
+    serde_json::to_value(item).map_err(|_| Error::Serialize)
+}
+
+pub fn from_item<I>(item: &Option<serde_json::Value>) -> Result<I>
 where
     I: DeserializeOwned,
 {
     if let &Some(ref item) = item {
-        serde_json::from_value(item.clone()).map_err(|_| Error::Deserialize)
+        from_value(item)
     } else {
         Err(Error::NotFound)
     }
 }
 
-pub fn get_vec<I>(v: &Vec<serde_json::Value>) -> Result<Vec<I>>
+pub fn to_item<I>(item: I) -> Result<Option<serde_json::Value>>
+where
+    I: Serialize,
+{
+    to_value(item).map(Some)
+}
+
+pub fn from_vec<I>(v: &Vec<serde_json::Value>) -> Result<Vec<I>>
 where
     I: DeserializeOwned,
 {
     v.iter().fold(Ok(Vec::new()), |acc, item| match acc {
-        Ok(mut acc) => match serde_json::from_value(item.clone()) {
-            Ok(item) => {
-                acc.push(item);
-                Ok(acc)
-            }
-            Err(_) => Err(Error::Deserialize),
-        },
-        Err(e) => Err(e),
+        Ok(mut acc) => from_value(item).map(|item| {
+            acc.push(item);
+            acc
+        }),
+        e => e,
+    })
+}
+
+pub fn to_vec<I>(v: Vec<I>) -> Result<Vec<serde_json::Value>>
+where
+    I: Serialize,
+{
+    v.into_iter().fold(Ok(Vec::new()), |acc, item| match acc {
+        Ok(mut acc) => to_value(item).map(|item| {
+            acc.push(item);
+            acc
+        }),
+        e => e,
     })
 }