diff --git a/serde/src/integer128.rs b/serde/src/integer128.rs
index 169c7128..71db2465 100644
--- a/serde/src/integer128.rs
+++ b/serde/src/integer128.rs
@@ -1,4 +1,75 @@
-/// TODO: rustdoc
+/// Conditional compilation depending on whether Serde is built with support for
+/// 128-bit integers.
+///
+/// Data formats that wish to support Rust compiler versions older than 1.26 may
+/// place the i128 / u128 methods of their Serializer and Deserializer behind
+/// this macro.
+///
+/// Data formats that require a minimum Rust compiler version of at least 1.26
+/// do not need to bother with this macro and may assume support for 128-bit
+/// integers.
+///
+/// ```rust
+/// #[macro_use]
+/// extern crate serde;
+///
+/// use serde::Serializer;
+/// # use serde::private::ser::Error;
+/// #
+/// # struct MySerializer;
+///
+/// impl Serializer for MySerializer {
+///     type Ok = ();
+///     type Error = Error;
+///
+///     fn serialize_i64(self, v: i64) -> Result<Self::Ok, Self::Error> {
+///         /* ... */
+/// #         unimplemented!()
+///     }
+///
+///     /* ... */
+///
+///     serde_if_integer128! {
+///         fn serialize_i128(self, v: i128) -> Result<Self::Ok, Self::Error> {
+///             /* ... */
+/// #             unimplemented!()
+///         }
+///
+///         fn serialize_u128(self, v: u128) -> Result<Self::Ok, Self::Error> {
+///             /* ... */
+/// #             unimplemented!()
+///         }
+///     }
+/// #
+/// #     __serialize_unimplemented! {
+/// #         bool i8 i16 i32 u8 u16 u32 u64 f32 f64 char str bytes none some
+/// #         unit unit_struct unit_variant newtype_struct newtype_variant seq
+/// #         tuple tuple_struct tuple_variant map struct struct_variant
+/// #     }
+/// }
+/// #
+/// # fn main() {}
+/// ```
+///
+/// When Serde is built with support for 128-bit integers, this macro expands
+/// transparently into just the input tokens.
+///
+/// ```rust
+/// macro_rules! serde_if_integer128 {
+///     ($($tt:tt)*) => {
+///         $($tt)*
+///     };
+/// }
+/// ```
+///
+/// When built without support for 128-bit integers, this macro expands to
+/// nothing.
+///
+/// ```rust
+/// macro_rules! serde_if_integer128 {
+///     ($($tt:tt)*) => {};
+/// }
+/// ```
 #[cfg(integer128)]
 #[macro_export]
 macro_rules! serde_if_integer128 {