In constant, a gas-phase reaction of CO, H2S, and H2 on molybdenum sulfide (MoS2) catalyst is a well-known artificial means of methanethiol production . This fact led us to the idea that methanethiol was sourced from subseafloor scCO2. Indeed, CO is generated from CO2 and H2 through the water-gas shift reaction (CO2 + H2 ⇌ CO + H2O) even in a water-filled condition . Mo is a trace but ubiquitous component of natural basalt and komatiite . Although the rock-containing Mo is predominantly in the +6 valence state (i.e., presents as molybdate (MoO4 2-)) , molybdate is readily converted to MoS2 with H2S as the reductant and source of sulfur (see Materials and Methods). It is widely recognized that the presence of water often severely suppresses catalytic reactions . However, we show below that, with MoS2 deposited in a carbonate-NaCl solution, methanethiol formation still effectively proceeds from H2S, H2, and scCO2 as a sole carbon source.