Thermal behaviour of polysiloxanes containing cyanoethyl side groups was studied by differential scanning calorimetry (DSC). Samples were quenched to - 165 degreesC and then reheated at different heating rates. The glass transition, an exothermal crystalline formation (cold crystallization) and endothermal melting of crystalline formations were detected. A broad cold crystallization peak and a melting peak were observed at a heating rate of 20 K/min. However, the samples gave a sharp cold crystallization peak and three endothermal peaks using a heating rate of 5 K/min. The DSC traces obtained were thus sensitive to heating rates. Thermal degradation of silazane polymers was studied by a thermogravimetric method. Three stages of degradation occur in the course of thermal decomposition. It was shown that the yields of pyrolytic product increased with the heating rate decreasing.