Undoubtedly, the most natural way to confirm a model is to quantitatively verify its predictions. However, this is not done systematically, and one of the reasons for that is the lack of appropriate tools for analyzing data, because the existing tools do not implement the required models or they lack the flexibility required to perform data analysis in a reasonable time. We present QSoas, an open-source, cross-platform data analysis program written to overcome these problems. In addition to standard data analysis procedures and full automation using scripts, QSoas features a very powerful data fitting interface with support for arbitrary functions, differential equation and kinetic system integration, and flexible global fits. QSoas is available from http://www.qsoas.org.