[Rod Vaughn]

Thanks – so they did remove some codec support.

I also found this related discussion on apple: https://discussions.apple.com/thread/5471254?start=0&tstart=0

As they suggested, I tried both adding the additional Apple Codecs in and removing Perian (which I forgot I even had installed). Neither worked for me, so I am still looking for a solution.

[Rod Vaughn]

Yep, you are probably right – all I know for sure is that I could play the MP4 files using the spacebar under Mountain Lion with no problem and now I can’t under Mavericks. Flip4Mac was one of the very few software updates I needed to do when I installed Mavericks and the only related to video, so I was hoping that was it. It wasn’t, so now my conclusion is that something definitely changed between the Mountain Lion and Mavericks OS.

[Rod Vaughn]

Thanks Don, I ran across the exact same problem (as you described it) with my upgrade to Mavericks from Mountain Lion as well. Including the attempted conversion from Quicktime. I thought it was because of my outdated version of Flip4Mac at first but an update to that didn’t solve the problem. Please let us know if you figure out how to get back the ability to preview the MP4s using the spacebar. I have years of MP4 and I would hate to loose the ability to preview/play them using the space bar. Converting them all is out of the question.