Twenty-three British, five American and one Spanish coal were analysed for their total sulphate content and the ratio of O-18 and O-16 isotopes in the sulphate molecule. One coal was also used to observe the effects of sulphate production on isotopic composition during the oxidation of coal in an aqueous slurry. The isotopic data for the 29 coals covered a wide range of values and the coals with higher sulphate contents showed some tendency towards more O-18-depleted values. This indicates a higher proportion of secondary sulphate derived from the weathering of sulphide minerals and as a result an oxygen isotopic composition dominated by groundwaters (O-18 depleted). Isotopic data derived from aqueous leaching experiments confirmed the effect which groundwaters have on controlling the sulphate isotopic composition during oxidation processes. However, this was not always the case with some high-sulphate coals having O-18-enriched compositions, indicating varying concentrations of primary sulphate. This method is therefore suitable for attributing variations in coal sulphate concentrations to either variable concentration of primary sulphate variation or differential addition of sulphate by secondary weathering processes.