When either H2S, H2, or MoS2 was absent, very little or no detectable methanethiol was formed (<0.1 μmol; Fig. ). The yield of methanethiol was negligibly low (<0.1 μmol) when MoS2 was replaced with mackinawite (FeS) or a synthetic Mo-free komatiite. Regarding the reported gas-phase production of methanethiol from CO, H2S, and H2 , the proposed catalytic process on MoS2 involves OCS formation from CO and H2S and the hydrogenation of OCS with H atoms provided by H2 dissociation. The necessity of H2 with H2S and MoS2 in our experiment (Fig. ), together with the detection of CO and OCS (Fig. and SI Appendix, Fig. ), suggests that a similar mechanism occurred in our binary system of CO2 and H2O. Additionally, because the run without H2S produced CH4 and CO in the greatest amounts (Fig. ), formation and sulfidation of the surface-bound methyl group are also potential intermediate steps to methanethiol. Interestingly, the coexistence of H2S, H2, and MoS2 also favored formate production (Fig. ). Formate is likely a byproduct, not an intermediate on the route to methanethiol: When 13 C-labeled formate (100 mmol L -1 ) was used as a starting material with H2S and MoS2 (H2 was not introduced in this run), the 7-day reaction at 200 °C led to the formation of a small amount of methanethiol (0.2 μmol); however, its mass spectrum was close to that of non-labeled standard (SI Appendix, Fig. ). A probable interpretation is that formate served as a source of H2 (HCOO -+ H + → CO2 + H2). As the byproduct 13 CO2 was significantly less than the amount of CO2 initially introduced (0.343 mmol at the maximum vs. ~150 mmol), its impact on the mass spectrum of methanethiol was negligible. Elucidation of the catalytic mechanism on MoS2 is beyond the scope of the present study, but further isotopic and kinetic assessment of the products, in combination with the spectroscopic/computational analyses of the surface process, should be an important future work.