Films of a polyimide siloxane copolymer in a thickness of 20 × 10-3 in have been characterized using thermal analysis. The thermal stability and the kinetics of decomposition have been investigated by thermogravimetry. The activation energy of decomposition was calculated to be about 1.7 eV. The approximate lifetime of the copolymer at different elevated temperatures was also estimated. The glass transition has been mutually examined by differential scanning calorimetry, thermomechanical analysis, and dynamic mechanical analysis. It was found that the glass transition temperature depends not only on heating rates but also on loading conditions. It can be derived that the mechanism of the glass transition in this copolymer involves two interconnected steps, namely, volume change and phase ordering. © 1990.