The curative waters of HCO₃-SO₄-Mg-Na+H₂S chemical type and 0.6-0.8 g/dm³ total dissolved solids are exploited in Horyniec Spa located at the northern boundary of the Carpathian Foredeep, near the Polish-Ukrainian border. Although these waters occur at relatively shallow depths (their aquifer is encountered about 30 m below the ground surface), they are quite well protected from surface pollutants by a cover of clayey sands. This paper revises and discusses the origin of specific chemical components of these waters based on the following analyses of stable isotopes: (a) δ³⁴S in SO₄²⁻, H₂S, and CaSO₄·2H₂O, (b) δ¹⁸O in SO₄²⁻, H₂O, and CaSO₄·2H₂O, and (c) δ²H in H₂O and CaSO₄·2H₂O. The most important process which takes place in the aquifer is the dissolution of gypsum present in the Miocene formation and subsequent bacterial reduction of sulphate ion to hydrogen sulphide in the presence of an organic matter. This process shifts both δ³⁴S and δ1¹⁸O of sulfate ions towards higher concentration of heavy isotopes by several per mil in comparison to that in the original Miocene gypsum. The hydrogen sulphide generated this way (up to a concentration of 45 mg/dm³) is highly depleted in a heavy isotope by about 41‰ with respect to a sulphate ion.