The free-radical copolymerization of vinyltrimethoxysilane (VTS) with vinylimidazole (VI) was carried out in benzene at 68 degrees C. Thermooxidative degradation and the SiOx film-formation mechanism of poly(VI-co-VTS) both on gold and KBr were studied with Fourier transform infrared (FTIR) spectroscopy. Reflection-absorption (RA) spectra on gold were compared with transmission spectra on KBr. In the initial stage of heat treatment, the Si--O--Si bond formation was caused mainly by the residual water in the copolymer film. As heating time increased, however, both the water resulting from the thermal decomposition of the copolymer and the water vapor in air began to participate in the hydrolysis of the Si--O--CH3 group followed by the condensation reaction. The structure of the SiOx film became closer to the structure of SiO2 with increasing heating temperature. In addition, the differences between the transmission spectra and R-A spectra were observed at the peaks related to Si--O--Si stretching due to the optical effect.