diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -949,6 +949,7 @@ class localrepository(repo.repository):
 
             # commit subs
             if subs or removedsubs:
+                pstate = subrepo.substate(self['.'])
                 state = wctx.substate.copy()
                 for s in sorted(subs):
                     sub = wctx.sub(s)
@@ -956,7 +957,19 @@ class localrepository(repo.repository):
                         subrepo.subrelpath(sub))
                     sr = sub.commit(cctx._text, user, date)
                     state[s] = (state[s][0], sr)
-                subrepo.writestate(self, state)
+
+                changed = False
+                if len(pstate) != len(state):
+                    changed = True
+                if not changed:
+                    for newstate in state:
+                        if state[newstate][1] != pstate[newstate]:
+                            changed = True
+                if changed:
+                    subrepo.writestate(self, state)
+                elif (changes[0] == ['.hgsubstate'] and changes[1] == [] and
+                     changes[2] == []):
+                    return None
 
             # Save commit message in case this transaction gets rolled back
             # (e.g. by a pretxncommit hook).  Leave the content alone on
diff --git a/mercurial/subrepo.py b/mercurial/subrepo.py
--- a/mercurial/subrepo.py
+++ b/mercurial/subrepo.py
@@ -13,6 +13,19 @@ hg = None
 
 nullstate = ('', '', 'empty')
 
+
+def substate(ctx):
+    rev = {}
+    if '.hgsubstate' in ctx:
+        try:
+            for l in ctx['.hgsubstate'].data().splitlines():
+                revision, path = l.split(" ", 1)
+                rev[path] = revision
+        except IOError as err:
+            if err.errno != errno.ENOENT:
+                raise
+    return rev
+
 def state(ctx, ui):
     """return a state dict, mapping subrepo paths configured in .hgsub
     to tuple: (source from .hgsub, revision from .hgsubstate, kind
@@ -39,15 +52,7 @@ def state(ctx, ui):
     for path, src in ui.configitems('subpaths'):
         p.set('subpaths', path, src, ui.configsource('subpaths', path))
 
-    rev = {}
-    if '.hgsubstate' in ctx:
-        try:
-            for l in ctx['.hgsubstate'].data().splitlines():
-                revision, path = l.split(" ", 1)
-                rev[path] = revision
-        except IOError, err:
-            if err.errno != errno.ENOENT:
-                raise
+    rev = substate(ctx)
 
     state = {}
     for path, src in p[''].items():
diff --git a/tests/test-subrepo-empty-commit.t b/tests/test-subrepo-empty-commit.t
new file mode 100644
--- /dev/null
+++ b/tests/test-subrepo-empty-commit.t
@@ -0,0 +1,64 @@
+  $ hg init
+  $ hg init sub
+  $ echo 'sub = sub' > .hgsub
+  $ hg add .hgsub
+  $ echo c1 > f1
+  $ echo c2 > sub/f2
+  $ hg add -S
+  adding f1
+  adding sub/f2
+  $ hg commit -m0
+  committing subrepository sub
+
+Make .hgsubstate dirty:
+
+  $ echo '0000000000000000000000000000000000000000 sub' > .hgsubstate
+  $ hg diff --nodates
+  diff -r 853ea21970bb .hgsubstate
+  --- a/.hgsubstate
+  +++ b/.hgsubstate
+  @@ -1,1 +1,1 @@
+  -5bbc614a5b06ad7f3bf7c2463d74b005324f34c1 sub
+  +0000000000000000000000000000000000000000 sub
+
+trying to do an empty commit:
+
+  $ hg commit -m1
+  committing subrepository sub
+  nothing changed
+  [1]
+
+an okay update of .hgsubstate
+  $ cd sub
+  $ echo c3 > f2
+  $ hg commit -m "Sub commit"
+  $ cd ..
+  $ hg commit -m "Updated sub"
+  committing subrepository sub
+
+deleting again:
+  $ echo '' > .hgsub
+  $ hg commit -m2
+  $ cat .hgsub
+  
+  $ cat .hgsubstate
+
+an okay commit, but with a dirty .hgsubstate
+  $ echo 'sub = sub' > .hgsub
+  $ hg commit -m3
+  committing subrepository sub
+  $ echo '0000000000000000000000000000000000000000 sub' > .hgsubstate
+  $ hg diff --nodates
+  diff -r 41e1dee3d5d9 .hgsubstate
+  --- a/.hgsubstate
+  +++ b/.hgsubstate
+  @@ -1,1 +1,1 @@
+  -fe0229ee9a0a38b43163c756bb51b94228b118e7 sub
+  +0000000000000000000000000000000000000000 sub
+  $ echo c4 > f3
+  $ hg add f3
+  $ hg status 
+  M .hgsubstate
+  A f3
+  $ hg commit -m4
+  committing subrepository sub