By comparing the observed non‐sea‐salt‐sulphate:sodium (nss‐SO42−:Na+) ratios with model calculations the main sources of nss‐SO42− observed in sea‐salt aerosol could be determined. As heterogeneous production appears to be responsible for at most half of the measured nss‐SO42− [Clegg and Toumi, 1997; Chameides and Stelson, 1992] other mechanisms have been examined here. Processes such as the presence of oxidizers other than hydrogen peroxide and ozone and alternative sources of sulphur appear to make little if any difference to calculated ratios. Contamination by “pure” sulphate particles is deemed unlikely, as is a sufficient increase in the alkalinity of the sea‐salt particles. However, observed ratios can be achieved in the presence of even a small amount of cloud processing. While observed ratios may possibly be due to experimental uncertainties the sulphate production produced by cloud processing is the most likely explanation. Predictions also show that smaller sea‐salt particles will have much greater nss‐SO42−:Na+ ratios than larger sea‐salt particles. The presence of large quantities of submicron sea‐salt particles which could also act as cloud condensation nuclei would make sea salt the largest sink for sulphur dioxide in the marine boundary layer.