Playing AVI and DivX files
This chapter from our book Troubleshooting Mac® OS X discusses why some Audio Video Interleaved (AVI) multimedia files do not play in QuickTime and provides methods for playing AVIs on Mac OS X.
There has been confusion over AVI files since QuickTime® added support for such. AVI is a media container very similar to QuickTime, but with a unique data format. AVI was originally called Video for Windows® (VFW) and QuickTime supports the VFW format.
However, most, if not all AVI playback issues arise with movies that have been compressed using CODECs (Compressor - Decompressor) that are unavailable for QuickTime in Mac OS X. In particular, the Indeo® CODEC, popular for video compression on PCs, has not been ported to Mac OS X.
To further confuse the issue, many DivX®-encoded files carry the .avi extension. QuickTime does not include native DivX support, even though QuickTime 6 and later support the ISO Standard MPEG-4 media compression format and DivX is based on the MPEG-4 standard.
QuickTime X and AVIs
Under Mac OS X 10.6 Snow Leopard®, QuickTime Player aka QuickTime X does not support third-party components, i.e. QuickTime plug-ins. Nevertheless, Snow Leopard includes QuickTime Player 7 as an optional install: QuickTime X will invoke QuickTime Player 7 to play videos supported by the latter or by QuickTime Player 7 components.
If you have an existing QuickTime Pro key, QuickTime Player 7 is installed automatically and Pro functionality is available in QuickTime Player 7. If you elect not to install QuickTime Player 7, it can be installed later. Likewise, if you attempt to open a media file that requires QuickTime Player 7, you are offered the opportunity to install it. QuickTime Player 7 is installed in the Macintosh HD > Applications > Utilities folder.
Playing AVIs on Mac OS X
We have found that the following four methods will enable viewing most AVI files that do not play in QuickTime. While none of these methods are foolproof and some AVI files still refuse to play, one of these methods will usually work.
Probably the easiest way to play AVI and DivX files on Mac OS X is to download and install the freeware VLC Media Player for OS X. VLC is also often referred to by its original name, VideoLAN Client.
Perian is an excellent, free, and open source QuickTime component.
Install the 3ivx® D4 CODEC
Download and install the 3ivx MPEG-4 CODEC per the installation instructions in the associated ReadMe file. This CODEC adds support for a variety of DivX and other formats which often carry the .avi extension.
Convert AVIs to QuickTime movies
If all else fails and you are running Mac OS X 10.6 or earlier, it may be possibles to convert the .avi files to QuickTime .mov files using tools from 3ivx.
You will need...
Try the DivX for Mac CODEC
Many AVIs can be played in QuickTime by installing the freeware DivX for Mac CODEC. While this plug-in has been continuously improved, the sound in some AVIs will sometimes be either missing or distorted. 
 Both the 3ivx D4 and DivX for Mac CODECs must be installed in the Macintosh HD > Library > QuickTime folder. However, both CODECs can not be in this folder simultaneously and work with QuickTime as they conflict. If you decide to keep both CODECs , you must temporarily disable one while using the other. To do this: