AVIAddXSubs is a free tool for adding SRT, IDX/SUB subtitles to AVI, MKV and MP4 format movies. The operation of the tool is very simple, just put in one folder both the movie and subtitles (required by the same name of both files), and then drag-and-drop the video file to the AVIAddXSubs icon. According to the creators' assurances, the conversion process takes very little time (from a few seconds to a few minutes), and during it there is no re-coding of the video, which allows to maintain the original quality of the material.

Importantly, subtitles are not superimposed on the image during conversion, but saved in a different format as a bitmap. The program offers the possibility of defining many mainly visual aspects of the font used during conversion. It allows, among other things, to define the font's cut and size, its colour and position on the screen.

The user is also provided with the option of previewing a sample of subtitles before encoding. AVIAddXSubs is available in a portable form that does not require installation.