Potassium-doped MoS2 catalysts supported on Al2O3 were synthesized, characterized by using atomic absorption spectroscopy, N2 physisorption, NO adsorption, X-ray diffraction, temperature-programmed sulfidation, and Raman spectroscopy, and tested in the synthesis of methanethiol from carbonyl sulfide (COS) and H2. The results revealed that two phases, pure MoS2 and potassium-decorated MoS2 (formed at high potassium loadings), were present in the active catalysts. The main effect of potassium during sulfidation and during the catalytic reaction was to increase the mobility of surface oxygen or sulfur atoms. Thus, potassium promoted the disproportionation of COS to CO2 and CS2 and the production of CO from CO2. Additionally, potassium cations hindered the reductive decomposition of COS to CO and H2S and the hydrogenolysis of methanethiol to methane. Mars–van Krevelen-type mechanisms were proposed to explain the disproportionation of COS on alumina and on the MoS2 phases. The catalytic site in the potassium-decorated MoS2 phase was proposed to include a potassium cation as adsorption site.