Calculations of the pH of sea salt aerosol particles show that the pH decreases with increasing relative humidity and therefore increasing liquid water content of the particles. We show that this puzzling effect is a result of the high concentration of chloride in the particles and the phase partitioning of HCl between the gas and aqueous phase, that makes the gas phase act as a reservoir for acidity for the sea salt particles. Implications for the marine boundary layer are briefly discussed. ¿ 2001 American Geophysical Union