Our study attempts to determine the influence of bacterial reduction on sulphate concentrations in groundwaters. A buried valley in a southeastern section of the Gwda River drainage basin was selected for fieldwork near the towns of Wysoka and Bądecz. Deep aquifers in the area are known for their high sulphate concentrations that result from pyrite oxidation by the Thiobacillus denitrificans bacteria. Isotope compositions of SO₄^2- ions show ³⁴S enrichment at lower sulphate concentrations, which might indicate bacterial reduction processes. Theoretical curves of SO₄^2- isotope composition and ion concentration change, drawn using a Rayleigh formula and the results of the determination of δ¹⁸O (SO₄^2-) and δ³⁴S (SO₄^2-) suggest widely varying intensities of the sulphate reduction processes. It can be estimated that bacterial reduction eliminated between 30% and in excess of 50% of the initial concentrations of SO₄^2- ions in individual cases.