Variation in sulphur isotope ratios is caused by equilibrium fractionation and kinetic fractionation. Two groups of samples show a narrow range of 34S/32S: sulphur from sea-water sulphate, and from meteorites and basic rocks; the former show an enrichment, δ34S, +20%0 with respect to the latter. They are identifiable source materials from which the complete range of values is derived by fractionation processes. Experimental and theoretical work shows the magnitude of these effects and allows interpretation of data from natural samples in terms of isotopic temperatures or source of the sulphur or both. Data mainly from rocks not associated with ore deposits is reviewed. The wide range of values in sediments can be attributed to bacterial reduction of sea-water sulphate under various conditions. Basalts give ratios near to the meteorite value, 0%0, and in detail relate to the conditions of petrogenesis. Some granites are enriched up to +30%0, probably from late-stage differentiation, but samples from the SE Autralian, New England batholith have ratios which fall into two groups: those within a small range from −3.6 to +5.0%0 and those which are generally higher or lower than that range. These respectively relate to an igneous or sedimentary source for the rocks as distinguished by other geological criteria.