Photoreactions for polysilane copolymers and terpolymers have been investigated and the resultant photochemical properties evaluated using deep-UV irradiation, differential photocalorimeter (DPC), FTIR and GPC. When exposed in the solid state to UV light, an increase of the molecular weights of the polymers was observed by GPC analysis of the soluble fractions. The reaction is highly exothermic as shown in the DPC spectra and this indicates that bond formation, or photocrosslinking is predominant during UV exposure in air. The photooxidation of the hydropolysilane thin films at high fluences of UV irradiation is limited by oxygen diffusion into the film. The UV photosensitivity of the polymers was evaluated by UV/VIS spectroscopy, on films that were exposed to varying amounts of UV energy. The examination of the photobleaching effect as a function of exposure energy reveals that the photobleaching rate increases with increasing Si-H content in the linear copolymers, and that the photobleaching rate also increases with increasing branch site content in the branched copolymers at a constant amount of Si-H component. Also, the alkyl-substituted polymer derivatives possess a higher photobleaching rate than the aryl-substituted polymers. The thermal sensitivity and degradation of a number of hydropolysilanes have been investigated by thermogravimetric analysis and FTIR spectroscopy in both air and oxygen free inert gas, such as helium or purified nitrogen, at temperatures up to 900 degree(s)C. The pyrolysis of hydropolysilanes in air is mainly attributed to a thermooxidative reaction and the products are predominantly SiOx, especially at higher temperatures. The polymers pyrolyzed under nitrogen yielded SiC-like products. The amount of SiC in the final products increases with increasing pyrolytic temperature as well as heating rate.