diff --git a/doc/hgrc.5.txt b/doc/hgrc.5.txt
--- a/doc/hgrc.5.txt
+++ b/doc/hgrc.5.txt
@@ -151,19 +151,27 @@ hooks::
   commit;;
     Run after a changeset has been created in the local repository.
     Passed the ID of the newly created changeset in environment
-    variable $NODE.
+    variable $NODE.  Parent changeset IDs in $P1 and $P2.
   incoming;;
     Run after a changeset has been pulled, pushed, or unbundled into
     the local repository.  Passed the ID of the newly arrived
     changeset in environment variable $NODE.
   precommit;;
-    Run before starting a commit.  Exit status 0 allows the commit to
-    proceed.  Non-zero status will cause the commit to fail.
+    Run before starting a local commit.  Exit status 0 allows the
+    commit to proceed.  Non-zero status will cause the commit to
+    fail.  Parent changeset IDs in $P1 and $P2.
   pretag;;
     Run before creating a tag.  Exit status 0 allows the tag to be
     created.  Non-zero status will cause the tag to fail.  ID of
     changeset to tag in $NODE.  Name of tag in $TAG.  Tag is local if
     $LOCAL=1, in repo if $LOCAL=0.
+  pretxncommit;;
+    Run after a changeset has been created but the transaction not yet
+    committed.  Changeset is visible to hook program.  This lets you
+    validate commit message and changes.  Exit status 0 allows the
+    commit to proceed.  Non-zero status will cause the transaction to
+    be rolled back.  ID of changeset in $NODE.  Parent changeset IDs
+    in $P1 and $P2.
   tag;;
     Run after a tag is created.  ID of tagged changeset in $NODE.
     Name of tag in $TAG.  Tag is local if $LOCAL=1, in repo if
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -376,7 +376,11 @@ class localrepository(object):
             self.ui.status(_("nothing changed\n"))
             return None
 
-        self.hook("precommit", throw=True)
+        xp1 = hex(p1)
+        if p2 == nullid: xp2 = ''
+        else: xp2 = hex(p2)
+
+        self.hook("precommit", throw=True, p1=xp1, p2=xp2)
 
         if not wlock:
             wlock = self.wlock()
@@ -462,13 +466,14 @@ class localrepository(object):
 
         user = user or self.ui.username()
         n = self.changelog.add(mn, changed + remove, text, tr, p1, p2, user, date)
+        self.hook('pretxncommit', throw=True, node=hex(n), p1=xp1, p2=xp2)
         tr.close()
 
         self.dirstate.setparents(n)
         self.dirstate.update(new, "n")
         self.dirstate.forget(remove)
 
-        self.hook("commit", node=hex(n))
+        self.hook("commit", node=hex(n), p1=xp1, p2=xp2)
         return n
 
     def walk(self, node=None, files=[], match=util.always):