The isotopic composition of the elemental, sulfate, pyritic and organic sulfur in a wide range of Australian coals has been determined. The S34 contents of the organic sulfur in coals containing more than 1 per cent total sulfur vary widely (dS34 + 2·9 to +24.4·)and contrast sharply with the narrow range of values found for the organic sulfur in coals of low total sulfur content (dS34 + 4·6 + 7·3). These latter values are believed to reflect the relative constancy in the isotopic composition of fresh water sulfate since the Permian. Where coal seams are overlain by marine deposits, isotopic measurements on sections through the seam allow: 1. (a) a clear distinction to be made between the secondary pyrite resulting from sea-water sulfate reduction and the primary pyrite, 2. (b) the depth of penetration by sulfate to be determined and 3. (c) interaction between the sulfur forms to be established, and also provide information regarding the mechanisms for sulfate reduction and for pyrite oxidation.