diff --git a/src/length.rs b/src/length.rs
index f2c89d3c..e14e7106 100644
--- a/src/length.rs
+++ b/src/length.rs
@@ -33,6 +33,7 @@ use std::fmt;
 /// another. See the `ScaleFactor` docs for an example.
 // Uncomment the derive, and remove the macro call, once heapsize gets
 // PhantomData<T> support.
+#[repr(C)]
 #[derive(RustcDecodable, RustcEncodable)]
 pub struct Length<T, Unit>(pub T, PhantomData<Unit>);
 
diff --git a/src/lib.rs b/src/lib.rs
index 09a1e3e5..55c98528 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -24,6 +24,9 @@
 //! Client code typically creates a set of aliases for each type and doesn't need
 //! to deal with the specifics of typed units further. For example:
 //!
+//! All euclid types are marked #[repr(C)] in order to facilitate exposing them to
+//! foreign function interfaces (provided the underlying scalar type is also repr(C)).
+//!
 //! ```rust
 //! use euclid::*;
 //! pub struct ScreenSpace;
diff --git a/src/macros.rs b/src/macros.rs
index 8984c4e1..26471a63 100644
--- a/src/macros.rs
+++ b/src/macros.rs
@@ -14,6 +14,7 @@ macro_rules! define_matrix {
             $(pub $field:ident: T,)+
         }
     ) => (
+        #[repr(C)]
         $(#[$attr])*
         pub struct $name<T, $($phantom),+> {
             $(pub $field: T,)+
diff --git a/src/scale_factor.rs b/src/scale_factor.rs
index bd004886..d41395ea 100644
--- a/src/scale_factor.rs
+++ b/src/scale_factor.rs
@@ -36,6 +36,7 @@ use std::marker::PhantomData;
 /// let one_foot: Length<f32, Inch> = Length::new(12.0);
 /// let one_foot_in_mm: Length<f32, Mm> = one_foot * mm_per_inch;
 /// ```
+#[repr(C)]
 #[derive(RustcDecodable, RustcEncodable)]
 pub struct ScaleFactor<T, Src, Dst>(pub T, PhantomData<(Src, Dst)>);