A geochemical history of the sulfur isotopes in the crust of the earth is presented. New measurements of the S 32 /S 34 ratio on sulfide minerals are used to show that transport and depositional processes do not appear to produce appreciable variation in the relative abundance of the sulfur isotopes. The average S 32 /S 34 ratio in an ore deposit as well as the range of values provides information on the ultimate source of the ore-forming fluids and to some extent the mechanism of emplacement. Galenas containing lead of normal isotopic composition generally have a narrow range of S 32 /S 34 ratio, whereas anomalous leads carry sulfur of highly variable composition. The S 32 /S 34 ratios in coexisting galena and barite can be used to estimate the temperature of formation of an ore deposition but criteria of equilibrium are difficult to establish.