Catalytic hydrothermal gasification (cHTG) is a sustainable and promising route to convert wet biomass into renewable methane, but the catalyst is deactivated by small amounts of soluble sulfate stemming from the biomass feedstock. Under hydrothermal conditions, sulfate can be converted to the strong catalyst poisons thiols or sulfides by thermochemical sulfate reduction (TSR). In this work, we explored the extent to which TSR occurs under conditions relevant to cHTG operation, along with the impact of various materials on the reaction. Our results indicated that within 60 min, an aqueous solution of 20 wt% glycerol and 10 mM potassium sulfate at 25 MPa started to produce organosulfur compounds at ca. 420–440 °C, with yields for the volatile ones of 3% and 6% at 440 °C and 490 °C, respectively. The main products were non-volatile organosulfur compounds (NVOSC), which were tentatively identified as sulfate esters of glycerol or of glycerol decomposition products. A smaller fraction of products consisted of volatile organosulfur compounds (VOSC). The VOSC identified from the TSR with glycerol were mostly alkyl thiophenes, as well as methanethiol, disulfides and trisulfides with no evidence of H₂S formed. Ni-, Mo-, or NiMo_x-based materials did not show any catalytic effect on sulfate reduction, independent of the sulfidation of the material. Glycerol was proven to be much more reactive towards TSR than acetone, as no TSR products could be observed when acetone was used as a reducing agent. A reaction pathway for the formation of thiophenes from H₂S and glycerol is proposed that consists of the dehydration of glycerol, the formation of CO and H₂ from glycerol decomposition, hydroformylation of acrolein, and the Paal-Knorr synthesis with succinaldehyde and H₂S as intermediates.