Highly mineralized springs in the Scuol-Tarasp area of the Lower Engadin and in the Albula Valley near Alvaneu, Switzerland, display distinct differences with respect to the source and fate of their dissolved sulphur species. High sulphate concentrations and positive sulphur (δ ³⁴S) and oxygen (δ ¹⁸O) isotopic compositions argue for the subsurface dissolution of Mesozoic evaporitic sulphate. In contrast, low sulphate concentrations and less positive or even negative δ ³⁴S and δ ¹⁸O values indicate a substantial contribution of sulphate sulphur from the oxidation of sulphides in the crystalline basement rocks or the Jurassic sedimentary cover rocks. Furthermore, multiple sulphur (δ ³⁴S, Δ ³³S) isotopes support the identification of microbial sulphate reduction and sulphide oxidation in the subsurface, the latter is also evident through the presence of thick aggregates of sulphide-oxidizing Thiothrix bacteria.