Thermal and mechanical properties of poly(diarylsilmethylene)s (PDArSMs) with phenyl, m-tolyl or p-tolyl substituents on Si atoms were investigated. According to the TG-DTA, poly(diphenylsilmethylene) (PDPSM) remained almost unchanged in weight up to about 400 degrees C, and showed two-step weight loss in the temperature range between ca. 400 and 700 degrees C in an air atmosphere. The first decomposition step seems assignable to elimination of aryl groups while the second one is presumably due to oxidative decomposition which is evident from a large exothermic peak in the DTA trace. Thermostability of PDArSMs with tolyl groups were comparable to that of PDPSM when the measurement was conducted under nitrogen, whereas the polymers having tolyl substituents were highly thermally unstable in air compared with PDPSM. These polymers exhibited a gain in weight accompanied with a violent exothermic reaction in the early stage of thermal decomposition in air. This thermal instability of the tolyl-substituted polymers can be explained by high sensitivity of the tolyl groups toward oxidative decomposition. Analysis of pyrolyzed products of PDArSMs suggested that these polymers underwent radical cleavage of Si-aryl bonds to provide network materials which can be precursors for Si-containing ceramics. Mechanical properties of PDPSM are also discussed.