diff --git a/src/ol/transform.js b/src/ol/transform.js
index 65c3b4f2f1..cbb19c3c39 100644
--- a/src/ol/transform.js
+++ b/src/ol/transform.js
@@ -237,6 +237,12 @@ export function determinant(mat) {
   return mat[0] * mat[3] - mat[1] * mat[2];
 }
 
+/**
+ * A string version of the transform.  This can be used
+ * for CSS transforms.
+ * @param {!Transform} mat Matrix.
+ * @return {string} The transform as a string.
+ */
 export function toString(mat) {
   return 'matrix(' + mat.join(', ') + ')';
 }