The thermal stability of non-F- (LPS-I) and F-(LPS-II) containing ladder polyepoxysilsesquioxanes under air and nitrogen was studied by DSC, TGA and TGA-FFIR techniques. The thermally curing temperature for LPS-II is about 20 degreesC higher than that for LPS-I. Both LPS-I and LPS-II exhibits higher thermal stability tinder air and nitrogen as observed from TGA, where the decomposition temperatures at 5 and 10% weight loss were above 425 and 480 degreesC, respectively. Under nitrogen, the higher decomposition temperature and residue from TGA was attributed to the gradual, cleavage of C-Si and G-C bonds and high stability of F-containing side-chains (non-broken). Because of the weight gain from the oxidation of silsesquioxanes (RSiO3/2) to silica (SiO2), and gradual cleavage of F-containing side-chains, LPS-II exhibits higher decomposition temperature under air environment than under nitrogen. These phenomena detected from TGA could be observed directly from TGA-FTIR stack plots. As combined techniques for the evolved gas analysis, TGA-FTIR played an important role in explaining the phenomena of degradation related to the change of polymer chain structures during heating.