diff --git a/src/DOM/XHR/FormData.js b/src/DOM/XHR/FormData.js
index 683f2f6..cada302 100644
--- a/src/DOM/XHR/FormData.js
+++ b/src/DOM/XHR/FormData.js
@@ -5,22 +5,12 @@ exports.newFormData = function () {
   return new FormData();
 };
 
-exports.appendString = function (form) {
-  return function (key) {
-    return function (val) {
-      form.append(key, val);
-      return {};
-    };
-  };
+exports.appendStringImpl = function (form, key, val) {
+  form.append(key, val);
+  return {};
 };
 
-exports.appendWithName = function (form) {
-  return function (key) {
-    return function (val) {
-      return function (name) {
-        form.append(key, val, name);
-        return {};
-      };
-    };
-  };
+exports.appendWithNameImpl = function (form, key, val, name) {
+  form.append(key, val, name);
+  return {};
 };
diff --git a/src/DOM/XHR/FormData.purs b/src/DOM/XHR/FormData.purs
index 4921393..9c172c4 100644
--- a/src/DOM/XHR/FormData.purs
+++ b/src/DOM/XHR/FormData.purs
@@ -1,30 +1,27 @@
-module DOM.XHR.FormData (FormDataValue(..), toFormData) where
+module DOM.XHR.FormData
+	( FormDataValue(..)
+	, toFormData
+	) where
 
 import Prelude
-import Data.Foldable (class Foldable, foldMap)
+import Control.Monad.Eff (Eff)
+import Control.Monad.Eff.Uncurried (EffFn3, runEffFn3, EffFn4, runEffFn4)
+import Control.Monad.ST (ST)
+import Data.Foldable (class Foldable, for_)
 import Data.Tuple (Tuple(..))
+import DOM (DOM)
 import DOM.File.Types (Blob, File)
-import DOM.XHR.Types (FormData)
+import DOM.XHR.FormData.ST (newFormData, appendString, appendWithName, freeze, thaw)
+import DOM.XHR.Types (FormData, FormDataValue)
 
--- | Possible values of a `FormData`.
-data FormDataValue = 
-	  FormDataString String
-	| FormDataFile String File -- ^ File name and `File` object.
-	| FormDataBlob String Blob -- ^ Blob name and `Blob` object.
+-- | Convert an associated array of keys and values to a `FormData`.
+toFormData :: forall eff f h. Foldable f => f (Tuple String FormDataValue) -> FormData
+toFormData dat = runPure $ runST $ do
+  result <- newFormData
 
--- | Convert an associated array of keys and values to a `FormData`. 
-toFormData :: forall f . (Foldable f) => f (Tuple String FormDataValue) -> FormData
-toFormData dat =
-	let form = newFormData unit in
-	let _unit = foldMap (appendData form) dat in
-	form
-	
-	where
-		appendData form (Tuple key (FormDataString val)) = appendString form key val
-		appendData form (Tuple key (FormDataFile name val)) = appendWithName form key val name
-		appendData form (Tuple key (FormDataBlob name val)) = appendWithName form key val name
+  for_ dat $ case _ of
+    Tuple key (FormDataString    val) -> appendString   result key val
+    Tuple key (FormDataFile name val) -> appendWithName result key val name
+    Tuple key (FormDataBlob name val) -> appendWithName result key val name
 
-foreign import newFormData :: Unit -> FormData
-
-foreign import appendString :: FormData -> String -> String -> Unit
-foreign import appendWithName :: forall a . FormData -> String -> a -> String -> Unit
+  freeze result
diff --git a/src/DOM/XHR/FormData/ST.js b/src/DOM/XHR/FormData/ST.js
new file mode 100644
index 0000000..c4e4ba6
--- /dev/null
+++ b/src/DOM/XHR/FormData/ST.js
@@ -0,0 +1,27 @@
+"use strict";
+
+exports.emptySTFormData = function () {
+  return new FormData();
+};
+
+exports.appendSTFormDataImpl = function (formData, name, value) {
+  formData.append(name, value);
+  return {};
+};
+
+exports.setSTFormDataImpl = function (formData, name, value) {
+  formData.set(name, value);
+  return {};
+};
+
+exports.freezeImpl = function (formData) {
+  var result = new FormData();
+
+  formData.forEach(function (value, key) {
+    result.append(key, value);
+  });
+
+  return result;
+};
+
+exports.thawImpl = exports.freeze;
diff --git a/src/DOM/XHR/FormData/ST.purs b/src/DOM/XHR/FormData/ST.purs
new file mode 100644
index 0000000..1a5f35b
--- /dev/null
+++ b/src/DOM/XHR/FormData/ST.purs
@@ -0,0 +1,36 @@
+module DOM.XHR.FormData.ST
+  ( newFormData
+  , emptySTFormData
+  , appendSTFormData
+  , freeze
+  , thaw
+  ) where
+
+-- | Create a new, empty FormData object that is ready for mutation.
+foreign import emptySTFormData
+  :: forall a eff h.
+     Eff (st :: ST h | r) (STFormData h a)
+
+foreign import appendSTFormDataImpl
+  :: forall a h r.
+     EffFn3 (st :: ST h | r) STFormData String a Unit
+
+appendSTFormData = runEffFn3 appendSTFormDataImpl
+
+foreign import setSTFormDataImpl
+  :: forall a h r.
+     EffFn3 (st :: ST h | r) STFormData String a Unit
+
+setSTFormData = runEffFn3 setSTFormDataImpl
+
+foreign import freezeImpl
+  :: forall a h r.
+     EffFn1 (st :: ST h | r) STFormData FormData
+
+freeze = runEff1 freezeImpl
+
+foreign import thawImpl
+  :: forall a h r.
+     EffFn1 (st :: ST h | r) FormData STFormData
+
+thaw = runEff1 thawImpl
diff --git a/src/DOM/XHR/Types.purs b/src/DOM/XHR/Types.purs
index 178442a..ab8e8a1 100644
--- a/src/DOM/XHR/Types.purs
+++ b/src/DOM/XHR/Types.purs
@@ -4,3 +4,9 @@ foreign import data XMLHttpRequestEventTarget :: Type
 foreign import data XMLHttpRequestUpload :: Type
 foreign import data XMLHttpRequest :: Type
 foreign import data FormData :: Type
+
+-- | Possible values of a `FormData`.
+data FormDataValue
+  = FormDataString String
+  | FormDataFile String File -- ^ File name and `File` object.
+  | FormDataBlob String Blob -- ^ Blob name and `Blob` object.