A thermodynamic equilibrium model was applied to study the interactions of gas-phase NH3, HNO3, and HCl with size-resolved aerosols and estimate aerosol pH in the remote marine boundary layer during the First Aerosol Characterization Experiment (ACE 1). Analysis of model results and field measurements indicates that accumulation-mode aerosols were probably in equilibrium with NH3, HNO3, and HCl simultaneously. The largest coarse-mode aerosols did not appear to be in equilibrium with HNO3, but may have been in equilibrium with NH3 and HCl. The estimated pH of accumulation-mode aerosols was 0--2, a function primarily of the amount of sulfate relative to sea salt present in that mode. By contrast, the estimated equilibrium pH of coarse-mode aerosols was 2--5, a function primarily of relative humidity and gas-phase HCl. Prior to exposure to HCl, the estimated pH of fresh sea spray aerosols was 7--9, a function primarily of relative humidity. Sensitivity tests showed that the drying of aerosols during the sampling process may have volatilized up to 30% of NH4+. ¿ 2000 American Geophysical Union