Trimethylamine has been used in the growth of compound semiconductors either through the formation of an adduct, as in trimethylamine allane, or as a possible growth reactant. The kinetics of the gas-phase decomposition and dominant reaction products relevant to these applications were determined in this study. The multistep thermal decomposition of trimethylamine in hydrogen (H-2 and D-2) in a laminar-flow tube reactor was studied by in situ mass spectroscopy and the analysis of isolated decomposition products. A dimethylamino radical and a methyl group are formed in a first-order Arrhenius-type decomposition that begins at similar to 525 and is complete at similar to 650 degrees C. Additional products of this first step were deuterated methane, CH3D, (in D-2) and ethane, C2H5. The activation energy is 50.8 kcal/mol. In subsequent steps the dimethylamino radical degrades through the series N-methylenemethylamine, (CH3)N = CH2, the imidoyl radical (CH3)N = (CH)-H-., and hydrogen cyanide, accompanied by loss of H-2, hydrogen radical, and methyl radical, respectively, as well as by the formation of unidentified solids.