Hi, This is very strange, and I already tried almost everything to make this work but without success. Basically, all MP4 files are showing like this on my MPlayerX: http://d.pr/i/qMiQ/3VAL1qc1 Sound is good, just the video is impossible to see anything. All other file extensions open fine in MPlayerX. I already uninstalled, installed again MPlayerX, I removed Perian, added again, but nothing worked. On my iMac, everything works fine, but not on my MacBook Air. Any idea how to solve this? Thanks to all.