Abstract Miocene evaporites of the Carpathian Foredeep host an interesting sulphates group of potash deposits, including about 20 sulphates minerals. The study of sulphur isotopic composition of 10 of the sulphates minerals from the Kalush–Holyh and Stebnyk potash deposits shows that only the basal Ca-sulphates (anhydrite) from the Kalush–Holyn potash deposits has δ 34 S values typical of Neogene marine evaporites (+21.0‰). Potash minerals related to the deposits (polyhalite, anhydrite, kainite, langbeinite and kieserite) show δ 34 S values from +15.28‰ to +17.54‰, and the weathering zone minerals (picromerite, leonite, bloedite, syngenite and gypsum) show values ranging from +14.73‰ to +18.22‰. The recorded depletion of sulphur isotopic composition of the salt minerals of potash deposits (and their weathering zone) was probably caused by one or more of the following isotope fractionation factors: bacterial reduction of sulphate, effect of crystallization and inflow of surface waters containing sulphates enriched in light sulphur isotopes due to pyrite oxidation. Accordingly, the observed sulphur isotopic composition of minerals from these potash deposits demonstrates the depletion of the original brines and continual inflow of new (concentrated) seawater. Similar sulphur isotopic composition of minerals from the potash deposits and their weathering zone points out that there was no significant sulphur isotope fractionation during weathering and in this case inflow of surface water has an insignificant influence on sulphur isotopic composition.