Thermochemical reduction of MgSO₄ by natural gas was carried out in the presence of water. Chemical and carbon isotopic variations were characterized with microcoulometry, gas chromatography, FT-IR, XRD and GC-C-IRMS. It was found that natural gas reacted with MgSO₄ to produce H₂S, CO₂, MgO and solid bitumen as the main products of thermochemical sulfate reduction (TSR). Gas dryness, gas souring index and the extent of reaction parameter positively correlated with temperature. The extent of acidulation and oxidation of natural gas were intensified as TSR proceeded. TSR has induced an evident carbon isotope fractionation in natural gas. Based on the experimental results, the reaction mechanism was tentatively investigated. It was found that the catalytic role of a metallic ion in the TSR process may be controlled by at least three factors: the charges and the radius of a metallic ion and the number of Lewis acid centers scatted on the surface of the sulfates. The last factor is largely determined by the amount of water in a TSR system. A small amount of water and metallic ions with more charges and short radius may effectively initiate the TSR.