diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,7 @@
+# 1.0.2
+
+* Make mapAccumWhile & mapAccumS strict in accumulator state [#18](https://github.com/fpco/conduit-combinators/pull/18)
+
 # 1.0.1
 
 * mapAccumWhile, mapAccumWhileM, mapAccumS
diff --git a/Data/Conduit/Combinators.hs b/Data/Conduit/Combinators.hs
--- a/Data/Conduit/Combinators.hs
+++ b/Data/Conduit/Combinators.hs
@@ -1513,7 +1513,7 @@
             loop seed'
 STREAMING(scanl, scanlC, scanlS, f x)
 
--- | 'mapWhile' with a break condition dependent on an accumulator.
+-- | 'mapWhile' with a break condition dependent on a strict accumulator.
 -- Equivalently, 'CL.mapAccum' as long as the result is @Right@. Instead of
 -- producing a leftover, the breaking input determines the resulting
 -- accumulator via @Left@.
@@ -1524,9 +1524,9 @@
 mapAccumWhileC f =
     loop
   where
-    loop s = await >>= maybe (return s) go
+    loop !s = await >>= maybe (return s) go
       where
-        go a = either return (\(s', b) -> yield b >> loop s') $ f a s
+        go a = either (return $!) (\(s', b) -> yield b >> loop s') $ f a s
 {-# INLINE mapAccumWhileC #-}
 STREAMING(mapAccumWhile, mapAccumWhileC, mapAccumWhileS, f s)
 
@@ -1780,9 +1780,9 @@
 mapAccumWhileMC f =
     loop
   where
-    loop s = await >>= maybe (return s) go
+    loop !s = await >>= maybe (return s) go
       where
-        go a = lift (f a s) >>= either return (\(s', b) -> yield b >> loop s')
+        go a = lift (f a s) >>= either (return $!) (\(s', b) -> yield b >> loop s')
 {-# INLINE mapAccumWhileMC #-}
 STREAMING(mapAccumWhileM, mapAccumWhileMC, mapAccumWhileMS, f s)
 
@@ -2027,8 +2027,8 @@
         else writeMutVar ref $! S idx' mv front
 {-# INLINE addE #-}
 
--- | Consume a source in a way piecewise defined by a controlling stream. The
--- latter will be evaluated until it terminates.
+-- | Consume a source with a strict accumulator, in a way piecewise defined by
+-- a controlling stream. The latter will be evaluated until it terminates.
 --
 -- >>> let f a s = liftM (:s) $ mapC (*a) =$ CL.take a
 -- >>> reverse $ runIdentity $ yieldMany [0..3] $$ mapAccumS f [] (yieldMany [1..])
@@ -2037,8 +2037,8 @@
 mapAccumS f s xs = do
     (zs, u) <- loop (newResumableSource xs, s)
     lift (closeResumableSource zs) >> return u
-    where loop r@(ys, t) = await >>= maybe (return r) go
-              where go a = lift (ys $$++ f a t) >>= loop
+    where loop r@(ys, !t) = await >>= maybe (return r) go
+              where go a  = lift (ys $$++ f a t) >>= loop
 {-# INLINE mapAccumS #-}
 
 -- | Run a consuming conduit repeatedly, only stopping when there is no more
diff --git a/Data/Conduit/Combinators/Stream.hs b/Data/Conduit/Combinators/Stream.hs
--- a/Data/Conduit/Combinators/Stream.hs
+++ b/Data/Conduit/Combinators/Stream.hs
@@ -314,14 +314,14 @@
 mapAccumWhileS f initial (Stream step ms0) =
     Stream step' (liftM (initial, ) ms0)
   where
-    step' (accum, s) = do
+    step' (!accum, s) = do
         res <- step s
         return $ case res of
             Stop () -> Stop accum
             Skip s' -> Skip (accum, s')
             Emit s' x -> case f x accum of
-                Right (accum', r) -> Emit (accum', s') r
-                Left   accum'     -> Stop accum'
+                Right (!accum', r) -> Emit (accum', s') r
+                Left   !accum'     -> Stop accum'
 {-# INLINE mapAccumWhileS #-}
 
 mapAccumWhileMS :: Monad m =>
@@ -329,16 +329,16 @@
 mapAccumWhileMS f initial (Stream step ms0) =
     Stream step' (liftM (initial, ) ms0)
   where
-    step' (accum, s) = do
+    step' (!accum, s) = do
         res <- step s
         case res of
             Stop () -> return $ Stop accum
             Skip s' -> return $ Skip (accum, s')
             Emit s' x -> do
                 lr <- f x accum
-                case lr of
-                    Right (accum', r) -> return $ Emit (accum', s') r
-                    Left   accum'     -> return $ Stop accum'
+                return $ case lr of
+                    Right (!accum', r) -> Emit (accum', s') r
+                    Left   !accum'     -> Stop accum'
 {-# INLINE mapAccumWhileMS #-}
 
 data IntersperseState a s
diff --git a/Data/Conduit/Combinators/Unqualified.hs b/Data/Conduit/Combinators/Unqualified.hs
--- a/Data/Conduit/Combinators/Unqualified.hs
+++ b/Data/Conduit/Combinators/Unqualified.hs
@@ -1128,7 +1128,7 @@
 scanlC = CC.scanl
 {-# INLINE scanlC #-}
 
--- | 'mapWhileC' with a break condition dependent on an accumulator.
+-- | 'mapWhileC' with a break condition dependent on a strict accumulator.
 -- Equivalently, 'CL.mapAccum' as long as the result is @Right@. Instead of
 -- producing a leftover, the breaking input determines the resulting
 -- accumulator via @Left@.
diff --git a/conduit-combinators.cabal b/conduit-combinators.cabal
--- a/conduit-combinators.cabal
+++ b/conduit-combinators.cabal
@@ -1,5 +1,5 @@
 name:                conduit-combinators
-version:             1.0.1.1
+version:             1.0.2
 synopsis:            Commonly used conduit functions, for both chunked and unchunked data
 description:         Provides a replacement for Data.Conduit.List, as well as a convenient Conduit module.
 homepage:            https://github.com/fpco/conduit-combinators