Concentrations of HNO3(g) and NH3(g) determined in the field were compared to predictions from aerosol equilibrium models. The products of HNO3(g) and NH3(g) concentrations measured under cool and humid nonfoggy conditions agreed in magnitude with predictions from a comprehensive thermodynamic model for the atmospheric H2SO4‐HNO3‐NH3‐H2O system. Observed concentrations of NH3(g) in fogs were generally consistent with those predicted at equilibrium with fog water, but important discrepancies were noted in some cases. These discrepancies may be due to fluctuations in fog water composition over the course of sample collection or to the sampling of nonfoggy pockets of air present within the fog. Detectable concentrations of HNO3(g) (up to 23 neq m−3) were often found in fogs withpH5 were below the detection limit of 4–