A series of new crosslinked poly[(methylsiloxane)-co-(dimethylsilazane)] copolymers were synthesized by cationic ring opening polymerization of the cyclic monomers 1,3,5,7 -tetramethylcyclotetrasiloxane and 2,2,4,4,6,6 -hexamethylcyclotrisilazane. These copolymers contain different concentrations of methylsiloxane and silazane co-monomer units. The thermal stability is strongly related to the concentration of methylsiloxane co-units in the copolymers. The weight loss curve of the copolymers lies in a temperature range between its two homopolymers. FT-IR studies are in good agreement with the proposed structure, either for the copolymers or for the residual powder ceramic obtained by pyrolysis. Two cured processes (thermal and UV) were used. The pyrolysis of the copolymers were carried out. under argon atmosphere at 1200degreesC and in air at 1000 degreesC. Kinetic decomposition parameters of the copolymers such as a pre-exponential factor the decomposition reaction orders and the activation energies were determined. The molecular weights were determined by osmometry.