diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 1.3.1
+
+* Add missing `SplitGen` instance for `StateGen`: [#183](https://github.com/haskell/random/pull/183)
+
 # 1.3.0
 
 * Improve floating point value generation and avoid degenerate cases: [#172](https://github.com/haskell/random/pull/172)
diff --git a/random.cabal b/random.cabal
--- a/random.cabal
+++ b/random.cabal
@@ -1,6 +1,6 @@
 cabal-version:      >=1.10
 name:               random
-version:            1.3.0
+version:            1.3.1
 license:            BSD3
 license-file:       LICENSE
 maintainer:         core-libraries-committee@haskell.org
diff --git a/src/System/Random/Internal.hs b/src/System/Random/Internal.hs
--- a/src/System/Random/Internal.hs
+++ b/src/System/Random/Internal.hs
@@ -127,8 +127,8 @@
 --
 -- Constructor is not exported, becasue it is important for implementation to enforce the
 -- invariant of the underlying byte array being of the exact same length as the generator has
--- specified in `System.Random.Seed.SeedSize`. Use `System.Random.Seed.mkSize` and
--- `System.Random.Seed.unSize` to get access to the raw bytes in a safe manner.
+-- specified in `System.Random.Seed.SeedSize`. Use `System.Random.Seed.mkSeed` and
+-- `System.Random.Seed.unSeed` to get access to the raw bytes in a safe manner.
 --
 -- @since 1.3.0
 newtype Seed g = Seed ByteArray
@@ -484,7 +484,7 @@
 -- @since 1.3.0
 uniformByteArray ::
      RandomGen g
-  => Bool -- ^ Should byte array be allocted in pinned or unpinned memory.
+  => Bool -- ^ Should byte array be allocated in pinned or unpinned memory.
   -> Int -- ^ Number of bytes to generate
   -> g -- ^ Pure pseudo-random numer generator
   -> (ByteArray, g)
@@ -610,7 +610,7 @@
 --
 -- @since 1.2.0
 newtype StateGen g = StateGen { unStateGen :: g }
-  deriving (Eq, Ord, Show, RandomGen, Storable, NFData)
+  deriving (Eq, Ord, Show, RandomGen, SplitGen, Storable, NFData)
 
 instance (RandomGen g, MonadState g m) => StatefulGen (StateGenM g) m where
   uniformWord32R r _ = state (genWord32R r)
@@ -950,7 +950,12 @@
   --
   -- There is a default implementation of 'isInRange' via 'Generic'. Other helper function
   -- that can be used for implementing this function are `isInRangeOrd` and
-  -- `isInRangeEnum`
+  -- `isInRangeEnum`.
+  --
+  -- Note that the @isRange@ method from @Data.Ix@ is /not/ a suitable default
+  -- implementation of 'isInRange'. Unlike 'isInRange', @isRange@ is not
+  -- required to be symmetric, and many @isRange@ implementations are not
+  -- symmetric in practice.
   --
   -- @since 1.3.0
   isInRange :: (a, a) -> a -> Bool