The reaction rate constant of Ar(P-3(2)) metastable atoms with (Si(CH3)(4) (tetramethylsilane or TMS molecule) has been studied in a microwave discharge afterglow by means of absorption spectroscopy measurements on Ar(P-3(2)) at the 811.5 nm transition line and mass spectrometry. Among the various molecules or radicals produced by the chemical reactive process Si(CH3)(x) radicals are the main species. They arise from SI-C bond breaking. H-2(+) is also detected. This proves that the SiCxHy species (with y < 3x) is also created by C-H bonds breaking. However, they are only weakly detected probably because of their low concentration. We have determined for the relative reaction branching ratio producing Si(CH3)(2) species: 5% Si(CH3)(2), 15% Si(CH3)(2) and 80% Si(CH3). Moreover we have measured a total de-excitation rate constant for the Ar(P-3(2)) metastable atoms reacting with tetramethylsilane, ranging from 6 X 10(-18) to 2.2 X 10(-17) m(3) s(-1). This result is compared to theoretical predictions.