The properties of aqueous ammonium salt solutions at the air/water surface are presented. By comparison of the properties of ammonium halides solutions with those of sodium halide, we note a close similarity, which indicates that the increments in surface tension may be explained in relation to the hydration enthalpy of anions. We find that the behaviors of the salts with nitrate anions which are polyatomic are also explained by hydration enthalpy in the same way. On the other hand, salts with ammonium cations which are polyatomic show deviation from a monotonous relationship between the surface tension increments and the hydration enthalpies observed for alkali metal halides. These results can be explained by ion pair formation in the surface region. (C) 2009 Elsevier Inc. All rights reserved