The pH of deliquesced sea-salt aerosol was derived from the Cl phase partitioning measured in marine air, corresponding meteorological conditions, and the thermodynamic properties of HCl. Under moderately polluted conditions at Bermuda during spring 1996, calculated sea-salt aerosol pH ranged from the mid-2s to the mid-3s. Cl- and NO3- concentrations in the lower half of the sea-salt size distribution were near thermodynamic equilibrium with respect to HClg and HNO3g; shorter aerosol lifetimes and slower reaction kinetics involving larger aerosol sustained moderate phase disequilibria. The active multiphase cycling of HCl buffered sea-salt aerosol pH to approximately the same value across the particle size spectrum. ¿ 1998 American Geophysical Union