In the present work, homopolymerization and copolymerization of methylcyclosiloxanes and methylcyclosilazanes are investigated. Homopolymers and copolymers produced are characterized using H-1 NMR and Fourier transform infrared spectroscopy. It is found that the reactivity of methylcyclosiloxanes is much lower than that of methylcyclosilazanes. The structures of the homopolymers and copolymers produced are discussed and it is suggested that the latter consists of a block structure containing siloxanes and silazanes in which the siloxane units are the smaller.