Sulphide springs in Slovenia are very rare and they form a peculiar feature in a carbonate setting. The basic geological, hydrogeochemical and isotopic characteristics of the Žveplenica sulphide dolomite spring (Trebuša Valley, NW Slovenia) were investigated, along with the geochemical and mineralogical characteristics of the sediment deposited at the spring location. Geological mapping defined a small, structurally and lithologically isolated dolomitic aquifer. The major groundwater geochemical composition is HCO₃ ⁻ > Mg²⁺ > Ca²⁺, indicating dissolution of dolomite. The concentration of SO₄ ²⁻ was very low. The groundwater was generally close to saturation with respect to calcite and dolomite. Geochemical modelling and other analyses indicated sulphur to originate not from gypsum and/or anhydrite, but from some other sources. We suggest the origin of sulphur be the dissolution of volcanogenic sulphidic enrichment in a highly variable, Ladinian clastic-limestone-volcanic rock association, forming the basis of early Carnian Cordevolian dolomite, from which the spring discharges. The measured δ¹³CDIC value of −11.9 ‰ indicates groundwater with a contribution of degraded organic matter and dissolved inorganic carbon in the aquifer. The isotopic composition of oxygen (δ¹⁸OH₂O), hydrogen (δDH₂O) and tritium was −8.1 ‰, δD −51.4 ‰ and ³H 3.64 TU, respectively. The δ¹⁸O and δD values indicated recharge from precipitation with H₂S exsolution, while the ³H activity shows groundwater older than 40 years. Mineralogical and geochemical analysis of sediment showed a typical carbonate (mostly dolomite) composition, which was in agreement with the geochemical and isotopic composition of the groundwater of the spring, indicating a deep sourced inorganic form of sulphur such as pyrite.