diff --git a/Library/Homebrew/os/mac/xcode.rb b/Library/Homebrew/os/mac/xcode.rb
index 3fdd90463047a403619ff8c1784fd8a7a826d9f3..3fa7d62aea2ec93af338bbfeda03bdd6d3fa75b6 100644
--- a/Library/Homebrew/os/mac/xcode.rb
+++ b/Library/Homebrew/os/mac/xcode.rb
@@ -158,8 +158,12 @@ module MacOS::CLT extend self
   # This is true ift he standard UNIX tools are present under /usr. For
   # Xcode < 4.3, this is the standard location. Otherwise, it means that
   # the user has installed the "Command Line Tools for Xcode" package.
+  # TODO: handle 10.9 CLT which has headers under:
+  # /Library/Developer/CommandLineTools/usr/include
   def installed?
-    MacOS.dev_tools_path == Pathname.new("/usr/bin")
+    return false if MacOS.version > :mountain_lion
+    MacOS.dev_tools_path == Pathname.new("/usr/bin") and
+      File.directory? "/usr/include"
   end
 
   def latest_version?
diff --git a/Library/Homebrew/superenv/macsystem.rb b/Library/Homebrew/superenv/macsystem.rb
index 7d2a7880f81fc934f370fd9d79a85dcac254fb68..7ac282b080d484d5338ff53d10ff2604ce7e19d7 100644
--- a/Library/Homebrew/superenv/macsystem.rb
+++ b/Library/Homebrew/superenv/macsystem.rb
@@ -1,7 +1,10 @@
 # new code because I don't really trust the Xcode code now having researched it more
 module MacSystem extend self
   def xcode_clt_installed?
-    File.executable? "/usr/bin/clang" and File.executable? "/usr/bin/lldb" and File.executable? "/usr/bin/make"
+    # TODO: handle 10.9 CLT which has headers under:
+    # /Library/Developer/CommandLineTools/usr/include
+    File.executable? "/usr/bin/clang" and File.executable? "/usr/bin/lldb" and
+      File.executable? "/usr/bin/make" and File.directory? "/usr/include"
   end
 
   def xcode43_without_clt?