Thin a-SiC:H films were grown by hot-wire chemical vapor deposition at 200 K on Si(100) using mono-, tri-, and tetramethylsilane as single source precursors. The film structure and thermal reactivity were compared using in situ multiple internal reflection Fourier transform infrared spectroscopy and temperature programmed reaction/desorption. The results indicate that both mono- and trimethylsilane precursors yield films containing mixed silicon hydrides, SiHx (x = 1-3), and mostly intact methyl groups. Tetramethylsilane did not lead to substantial film, growth. These results are consistent with a mechanism for film growth involving Si-H bond cleavage. All the films are stable to above 550 K. By 600 K, silane and methylsilanes evolve, following the loss of terminal SiH3 and Si(CH3)(x)H3-x groups in the films. At higher temperatures, hydrogen desorption and hydrocarbon evolution are observed. For films grown with monomethylsilane, methane is the main hydrocarbon evolved, but films grown with trimethylsilane yield mainly acetylene. The production of acetylene is consistent with the higher concentration of methyl groups in the trimethylsilane films.