In this paper we consider deep waters from horizons located under the Baltic-Sea bottom. The samples were taken twice from wells bored in Hel peninsula. The δ18O and δD values of these waters show their glacial origin since they have significantly low values (δ18O from −14 to −11 whereas the modern groundwaters in Gdańsk area have −10%0). In contrast to surface waters the sulphates dissolved in these deep waters have rather high and uniform δ18O values which are correlated with δ18O of H2O but not correlated with δ34S. The isotope patterns may be explained assuming that the waters are very old, in which the slow process of oxygen isotope exchange at low temperatures, but extended in time, could enrich the sulphate in heavier oxygen isotopes.