Elemental sulfur, as a transient intermediate compound, by-product, or catalyst, plays significant roles in thermochemical sulfate reduction (TSR) reactions. However, the mechanisms of the reactions in S–H2O–hydrocarbons systems are not clear. To improve our understanding of reaction mechanisms, we conducted a series of experiments between 200 and 340 C for S– H2O–CH4, S–D2O–CH4, and S–CH4–1m ZnBr2 systems in fused silica capillary capsules (FSCCs). After a heating period ranging from 24 to 2160 h (hrs), the quenched samples were analyzed by Raman spectroscopy. Combined with the in situ Raman spectra collected at high temperatures and pressures in the S–H2O and S–H2O–CH4 systems, our results showed that (1) the disproportionation of sulfur in the S–H2O–CH4 system occurred at temperatures above 200 C and produced H2S, SO4 2, and possibly trace amount of HSO4 ; (2) sulfate (and bisulfate), in the presence of sulfur, can be reduced by methane between 250 and 340 C to produce CO2 and H2S, and these TSR temperatures are much closer to those of the natural system (200 C that can be reached by deep drilling for hydrocarbon resources.