Sulphur isotope measurements made on aerosol and precipitation samples collected at three sites in Wales and Mace Head, Eire, between March 1993 and September 1994, had highest non-sea-salt sulphate Δ34S values, up to 11.9? for aerosol and 9.7? for precipitation, during the summer months. Aerosol methanesulphonate shows a clear summer peak with values up to 3--4 nmol m-3. Assuming samples contained only sulphur from terrestrial/anthropogenic, sea salt, and marine biogenic (from the oxidation of dimethylsulphide) sources, we used the sulphur isotope values to determine the contribution of dimethylsulphide-derived sulphate to total sulphate. In general the biogenic contribution was small, <10%, although in samples from marine air masses it was as high as 40%. A comparison of aerosol and precipitation Δ34S values found no evidence for large isotopic fractionations during the oxidation of sulphur dioxide. ¿ 1998 American Geophysical Union