diff --git a/Data/Conduit/List.hs b/Data/Conduit/List.hs
--- a/Data/Conduit/List.hs
+++ b/Data/Conduit/List.hs
@@ -510,7 +510,7 @@
 {-# INLINE concatMapMC #-}
 STREAMING(concatMapM, concatMapMC, concatMapMS, f)
 
--- | 'concatMap' with an accumulator.
+-- | 'concatMap' with a strict accumulator.
 --
 -- Subject to fusion
 --
@@ -535,7 +535,8 @@
 {-# DEPRECATED scanlM "Use mapAccumM instead" #-}
 
 -- | Analog of @mapAccumL@ for lists. Note that in contrast to @mapAccumL@, the function argument
---   takes the accumulator as its second argument, not its first argument.
+--   takes the accumulator as its second argument, not its first argument, and the accumulated value
+--   is strict.
 --
 -- Subject to fusion
 --
@@ -544,7 +545,7 @@
 mapAccumC f =
     loop
   where
-    loop s = await >>= maybe (return s) go
+    loop !s = await >>= maybe (return s) go
       where
         go a = case f a s of
                  (s', b) -> yield b >> loop s'
@@ -559,7 +560,7 @@
 mapAccumMC f =
     loop
   where
-    loop s = await >>= maybe (return s) go
+    loop !s = await >>= maybe (return s) go
       where
         go a = do (s', b) <- lift $ f a s
                   yield b
@@ -583,7 +584,7 @@
 scanM :: Monad m => (a -> b -> m b) -> b -> ConduitM a b m b
 INLINE_RULE(scanM, f, mapAccumM (\a b -> f a b >>= \r -> return (r, r)))
 
--- | 'concatMapM' with an accumulator.
+-- | 'concatMapM' with a strict accumulator.
 --
 -- Subject to fusion
 --
diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,3 +1,7 @@
+## 1.2.5
+
+* mapAccum and mapAccumM should be strict in their state [#218](https://github.com/snoyberg/conduit/issues/218)
+
 ## 1.2.4.1
 
 * Some documentation improvements
diff --git a/conduit.cabal b/conduit.cabal
--- a/conduit.cabal
+++ b/conduit.cabal
@@ -1,5 +1,5 @@
 Name:                conduit
-Version:             1.2.4.3
+Version:             1.2.5
 Synopsis:            Streaming data processing library.
 description:
     Hackage documentation generation is not reliable. For up to date documentation, please see: <http://www.stackage.org/package/conduit>.