Sulphur isotope ratios were measured in minerals from altered pyrite concretions from Israel. The pyrite and associated elemental sulphur have very similar isotopic composition (delta S 34 av. = -0.6 per mil). The gypsum and iron sulphates are isotopically lighter (av. = -9 per mil). It is suggested that both the sulphur and the sulphate were formed by oxidative processes, very probably microbiological.