A new thermodynamic equation derived from the integration of Gibbs adsorption equation for binary systems at the vicinity of the surface saturation region is presented. The equation, attached to a given solution model for describing the dependency on bulk composition of the solute activity coefficient, allows for the determination of the constants of the solution model and therefore for the activity coefficients by means of a nonlinear fitting of surface tension data. The model was tested for 19 aqueous binary systems at 298.15 K or 303.15 K, including alcohols, glycols, alkanolamines, carboxylic acids, acetone, and 1,4-dioxane, and for 5 non-aqueous systems with hexane or heptane plus ethanol or 1-propanol, and cyclohexane plus isooctane. The comparison of the infinite dilution activity coefficients with those obtained from a previously reported bulk-surface equilibrium model in which Volmer's surface equation of state is used, and with activity coefficients reported in the literature determined from liquid-vapor equilibrium, shows an improvement in the determination of activity coefficients from surface tension data. (C) 2012 Elsevier By. All rights reserved.