• TSR accelerated oil and hydrocarbon gas decompositions in the pyrolysis of OM. • Pyrite decomposition was responsible for the 34S-depletion of H 2 S at 330–450 °C. • TSR led to the enrichment in 34S of H 2 S and oils in thermal maturation of OM. • Two parts of 34S fractionation by KIE and 34S exchange were observed for TSR. Inorganic sulfur (S) species including pyrite (FeS 2) and sulfates may co-exist with organic matter (OM) in source rocks. Their inter-related effects on hydrocarbon generation and decomposition, and the 34S isotope fractionation during thermal maturation remain unclear. In this study, four groups of hydrothermal experiments (kerogen with pyrite, kerogen with pyrite and gypsum, kerogen with pyrite removal, and kerogen with pyrite removal and gypsum) were conducted at 330–450 °C and 50 MPa using a gold-tube system. These experiments showed that pyrite and gypsum had limited effect on the determined vitrinite reflectance (%Ro) and H/C of kerogens under hydrothermal conditions. However, the presence of gypsum led to the occurrence of TSR accelerating the decompositions of oil and hydrocarbon gases. TSR also resulted in the apparent increase of gas dryness and sourness, and the enrichment in 13C and 2H of methane. Experimental data confirmed that an equilibrium isotope effect (EIE) was responsible for the small 34S fractionation between H 2 S and its precursor OM-S during thermal cracking of OM-S. The higher yields and more negative 34S isotopic ratios (δ34S) of H 2 S in the pyrolysis of kerogen with FeS 2 revealed that the decomposition of 34S-depleted FeS 2 contributed to H 2 S generation at elevated temperatures. Additionally, δ34S of pyrolysis products (i.e., oil, H 2 S and residual kerogens) become much more enriched with TSR. Mass balance calculations suggested that the evolution of δ34S of H 2 S from TSR in closed systems proceeded in two stages: the kinetic isotope effect (KIE) dominates the 34S fractionation in the early stage of TSR; and 34S exchange between sulfate and H 2 S is more influential in the latter/higher T stage of TSR. These conclusions may provide additional insights for understanding of 34S isotope fractionation both in hydrothermal settings and in organic-rich shale with multiple S sources. [ABSTRACT FROM AUTHOR]