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Abstract: Hydrogen sulfide (H2S) is found in low concentration (less than 1%) as the undesirable component in Permo-Triassic evaporite/carbonate successions of the South Pars gas field, the largest gas accumulation in the world. Different isotopic analyses have been carried out on gas samples; gas condensates, and solid sulfate together with petrography to assess the origin of H2S. Anhydrite δ 34S changed from about +10.1 to about +29.3‰ from the Permian to the Triassic, respectively, confirming the Phanerozoic sulfur curve and showing that diagenesis has generally not destroyed the initial sulfur isotope stratigraphy of anhydrite. The H2S δ 34S is −5‰ in average in all the reservoir units. Reservoir is presently buried to depths ranging from 2600 to 3500m, representing present-day temperatures from 85 to 105°C (185 to 221°F). Hence, bacterial sulfate reduction (BSR) is the unlikely source because reservoir temperature is high for bacteria to be survived and H2S isotope signature does not show an effect of isotopic fractionation by bacteria. For the South Pars gases, Silurian''s hot shale of the Sarchahan Formation is postulated as major source rock. Biomarker analysis of the gas condensate indicates the suboxic to oxic depositional environment for the source rock has been dominated that couldn''t have noticeable organic sulfur. Long migration pass way of the gas and activity of the H2S and reaction with the carrier rocks cannot allow highly the contribution of the H2S to the gas reservoir by thermal decomposition of the organic matter. Moreover, the isotopic analyses deny the thermal decomposition of the organic matter for the H2S in the South Pars gas field. However, in the South Pars field, δ 34S hydrogen sulfide is equal to −5‰, which is 15‰ lower than Permian solid sulfate (+10‰) and can be resulted by thermochemical sulfate reduction (TSR) of the Permian anhydrite at relevant temperature regime (100°C) during a long period of the geological time. The low concentration of the H2S can be indicated by the starting phase of TSR reactions. [Copyright &y& Elsevier]