Vein-type and bedding-concordant mesothermal (180–410 °C) stibnite–sulphosalt mineralization at Arnsberg, NE Rhenish Massif, Germany, is hosted by Carboniferous pyrite-rich black shales and siliceous limestones. A detailed sulphur isotope study of the stibnite–sulphosalt mineralization and pyrite from a variety of regional host-rock lithologies has been carried out using an in situ laser combustion technique. The δ34S values of stibnite of various textural types are distinctly negative and lie in a narrow range between −23.9‰ and −17.1‰ (mean −20.1‰). In contrast, regional sedimentary–diagenetic pyrites display a large variation of their δ34S values between −45.4‰ and +9.3‰. There is little evidence for significant modification of the hydrothermal fluid during deposition and the S isotope signatures suggest that the sulphur of the stibnite mineralization was not locally derived. The δ34S values of pyrite in Givetian shales display a significantly narrower range of −28.2‰ to −7.5‰ and their mean composition (−17.1‰) is close to the δ34S values of the Arnsberg stibnite deposits. Considering the temperature-dependent isotopic fractionation between stibnite and reduced sulfur species, the δ34S values of the mineralizing fluid (−16.8‰; 200 °C) and the Givetian rock source are essentially identical. Therefore, we propose a model of leaching and isotopic homogenization of sulphur from the Middle Devonian shales and a subsequent northward migration of these fluids. The fluids were trapped in permeability-controlled positions within anticlinal zones, where fluid cooling induced deposition of stibnite and sulphosalts.