A parameterisation of internal mixing between sulphate and sea-salt aerosol is developed to determine the available externally mixed sulphate cloud condensation nuclei (CCN) population. This parameterisation is then combined with a multi-component aerosol-cloud parameterisation to predict cloud droplet concentration incorporating the physical competition between sea-salt and sulphate nuclei in the cloud nucleation processes. The results of the combined parameterisation indicate a significantly reduced role, compared to previous estimates, for sulphate in cloud droplet nucleation, and consequently, in indirect radiative forcing. However, the results also imply that cloud droplet concentration, and consequently, cloud albedo, has a greater susceptibility to change resulting from further anthropogenic SO2 emissions. ¿ 1999 American Geophysical Union