A series of new crosslinked copolymers was obtained by cationic ring-opening copolymerization of 1,3,5,7-tetramethylcpclotetrasiloxane and 1,3,5-trioxane. Theqoly[(methylsiloxane)-co-(oxyme thylene)] copolymers are white crystals or glassy. Crosslinking reactions take place in the reaction medium. The degree of crosslinking increases with a higher concentration of the methylsiloxane units in the copolymers. The combination of solid-state magic-angle spinning nuclear magnetic resonance of Si-29 and C-13 and Fourier-transform infrared spectroscopy provided a quantitative description of the copolymer microstructure.