Thermally stable polysilylenemethylenes (PSMs) with siloxane crosslinking moieties were successfully synthesized by chlorodephenylation of preformed poly(methylphenylsilylenemethylene) (PMPSM) and subsequent in situ alcoholysis/hydrolysis/condensation reactions. The simplified process and mild reaction conditions are quite advantageous. The crosslink density of these materials can be adjusted by the degree of chlorodephenylation, although an alkoxysilyl group remains to some extent. The resulting crosslinked PSMs have well defined structures in which the backbone is composed of MePhSiCH2 and Ale(MeC)SiCH2 as well as Me(O-1/2)SiCH2 as a crosslinking moiety. The resulting crosslinked PSMs exhibited glass-transition temperatures ranging from 15 to 20 degreesC, whereas that of linear PMPSM was 22 degreesC. The crosslinked PSMs remained unchanged in weight below 300 degreesC, suggesting that they are thermally stable up to that temperature, The good solvent resistance caused by crosslinking as well as high thermal stability of these materials allow us to design new PSM-based polymer blends and preceramic polymers.