Thanks Tom for your reply. But unfortunately, that was not the problem. I completely deleted all the Arduino software and downloaded and installed 1.0.3 and re-downloaded the X10h zip file and re-installed it. But I still get the same error as before. Running your X10Blink programs compiles OK but does not run. It hangs on the x10.write command. I am about to give up on the x10 library.