This codec for Quicktime is todays necessity for most Mac users, without it Quicktime is around about useless at handling any of todays .avi formatted movies. It is also the codec of choice for encoding to a lot of new "movie producers" with good quality output. peterpayne this is not a program/app. but rather an additional codec. I don't know anything about "Mactheripper" (only have a 233 iMac so not into DVD ripping etc.), but I assume it outputs in mpeg in which case you could use Quicktime PRO if you have already purchased that to export it to another format selecting 3ivx as the codec via the options button in the QT dialog box. If you don't have the PRO version of Quicktime you can use a program/application called MPEG Streamclip that is a free download and does much the same thing as Quicktime PRO in the export of .mpegs. If you are still none the wiser the paperwork that comes with the 3ivx download has a link to their website that has some excellent friendly help forums that should be able to assist you.