The bonding and thermal reactivity of thin a-SiC:H films have been studied and compared with that of methyl groups on single-crystalline Si and in thick polymer films. The films were deposited on silicon substrates at 200 K by hot-wire chemical vapor deposition (CVD) using methylsilane (CH3SiH3) as the precursor. The resulting films were probed by in situ multiple internal reflection-Fourier transform infrared (MIR-FTIR) spectroscopy, and the thermal decomposition products were measured by temperature-programmed reaction/ desorption (TPR/D). According to MIR-FTIR measurements, hydrogen is present in the films in the form of mixed silicon hydride species (SiH, SiH2, and SiH3) and intact methyl groups. TPR/D and MIR-FTIR annealing studies following growth at 200 K indicate that the film is stable up to 550 K. Above 550 K, a number of thermal etching processes are observed, leading to silane and methylsilane evolution at 600 K, followed by the loss of methane and molecular hydrogen by 780 K. Annealing studies show that the SiH3 species are more reactive at thermal energies than are methyl groups. The stability of methyl groups and the silicon mono-and dihydride groups in the films largely parallels that observed on single-crystalline silicon, but more reaction pathways are available in the thin films. Mechanisms for the film growth and thermal decomposition are proposed.