ESR and XPS are used to study the Mo-based catalysts MoO3/K2CO3/SiO2 and K2MoO4/SiO2 prepared with two kinds of precursors, (NH4)(6)Mo7O24. 4H(2)O and K2MoO4. The catalytic properties of the catalysts for methanethiol synthesis from high H2S-containing syngas are explored. The activity assay shows that the two catalysts have much the same activity for the reaction. By the ESR characterization of both functioning catalysts, the resonant signals of "oxo-Mo(V)" (g = 1.93), "thio-Mo(V)" (g = 1.98) and S (g = 2.01 or 2.04) can be detected. In the catalyst MoO3/SiO2 modified with K2CO3, as increasing amounts of K2CO3 are added, the content of "oxo-Mo(V)" increases, but "thio-Mo(V)" decreases. The XPS characterization indicates that Mo has mixed valence states of Mo4+, Mo5+ and Mo6+, and that S includes three kinds of species: S2- (161.5 eV), [S-S](2-) (162.5 eV) and S6+ (168.5 eV). Adding K2CO3 promoter to the catalysts, the Mo species of high valence state is easily sulphided and reduced to Mo2S and "oxo-M(V)", and the derivation of [S-S](2-) and S2- species from S is promoted simultaneously. The methanethiol synthesis is favored if the mole ratio of (Mo6+ + Mo5+)/Mo4+ less than or equal to 0.8 and S2-/[S-S](2-) is kept at a value of about 1.