Heterogeneous sulfate production in the remote marine boundary layer is studied by combining measurements collected during the first Southern Hemisphere marine Aerosol Characterization Experiment (ACE 1) with two numerical models: one simulating heterogeneous sulfate production in sea-salt aerosol water (SSAW) and the other simulating cloud processing. The models calculate oxidation of SO2 by O3 and H2O2 via aqueous-phase reactions both in SSAW and in cloud droplets. Model calculations indicate that as much as 50–75% of the observed non-sea-salt (NSS) sulfate in particles in the ambient diameter size range of 0.9–16 μm during the ACE 1 measurement period is due to production in SSAW, with cloud processing producing the remainder. The initial alkalinity of the wind-generated sea-salt aerosols can strongly influence sulfate production. Changing the effective alkalinity of SSAW from a value typical of seawater to a value based on the measurements taken at Cape Grim results in a better match to the observed NSS sulfate size distribution with up to 75% of the predicted supermicrometer NSS sulfate being due to heterogeneous conversion in SSAW. The average rates of removal of sulfate due to dry and wet deposition for the 0.4–16 μm size range are 38 and 14 ng m−3 d−1 respectively.