Application of the RF (radio frequency) cold plasma method to the decomposition of methanethiol (methyl mercaptan, GH(3)SH) at different O-2/CH3SH ratios (0-4.5), with various input powers (20-90 W), and at constant operating pressure (30 Torr) was investigated. The species detected in the CH3SH2/O-2/Ar RF plasma were SO2, CS2, OCS, CO, CO2, CH4, C2H4 C2H2, H-2, H2O, HCOH, and CH3OH. However, CS2, CH4, C2H4, C2H2, H-2, H2S, CH3SCH3 (DMS), and CH3S2CH3 (DMDS) were detected in the CH3SH/Ar RF plasma. In the CH3SH/Ar plasma, over 83.7% of the total sulfur input was converted into CS2 at 60 W; this is due to the lack of competition between O and S and the thermodynamic stability of CS2. In the oxygen-rich conditions of the CH3SH/O-2/ Ar plasma, the most predominant sulfur-containing compound was SO2. As the feed O-2/CH3SH ratio was increased, M-SO2 was increased, while M-CS2 was decreased simultaneously. M-OCS was reduced by increasing either the O-2/CH3SH ratio or the applied power. From the decay of CS2 and the generation of CO at a lower O-2/CH3SH ratio of 0.6, CS, CS2, and CO were suggested as the primary species to react with Q, OH, Oz, S, or St and then to form OCS. This study provides useful insight into the reaction mechanisms involved in the decomposition of CH3SH and, mainly, the formation of CS2, CH4, C2H2, C2H2, SO2, CO, CO2, and OCS in CH3SH/Ar and CH3SH/O-2/Ar plasmas.