The gas-phase reaction thermodynamics in the chemical vapor deposition system of preparing silicon carbide via methyltrichlorosilane pyrolysis is investigated with a relatively complete set of 226 species, in which the thermodynamic data of 163 species are evaluated in this work with accurate model chemistry G3(MP2) and G3//B3LYP calculations combined with standard statistical thermodynamics. The data include heat capacity (C _p,m^θ), entropy (S _m^θ), enthalpy of formation (Δ_f H _m^θ) and Gibbs free energy of formation (Δ_f G _m^θ). All the results are consistent with the available reliable experiments. Based on these thermodynamic data, the equilibrium concentration distribution of the 226 possible species in 300–2,000 K is evaluated with the chemical equilibrium principle under a typical experimental condition. It is shown that the theoretical results are in very good agreement with the experiments. We conclude that the present work is instructive for experiments with different conditions.