Calculation results for surface tension, surface concentration, and the relative Gibbs adsorption isotherm of binary systems, as a function of concentration and temperature, were obtained using a formal thermodynamic model, which includes the activity coefficients for both, the bulk and the surface layer. To take into account the non-ideality of the liquid bulk as well as that of the surface layer, the UNIFAC group contribution activity coefficient model, with binary interaction parameters derived from vapor-liquid equilibrium data and reported previously in the open literature, is also used. The calculation method used to derive the mixture surface tension and surface concentration, makes an analogy between pressure and vapor concentration from a traditional isothermal liquid-vapor bubble point calculation and the surface tension and surface concentration, as used here. Values for the surface tension of 31 binary systems, at different temperatures, covering the full concentration range, which makes a total of 105 different binary systems, were estimated. Simultaneously, values for the surface concentration for the same binary systems were also calculated, which in turn were used to derive values of the relative Gibbs adsorption isotherm, employing a new scheme proposed in this work. The binary systems included in this study, have representatives from different chemical species which include polar, non-polar, and associating compounds. The average relative percent error obtained from the comparison of experimental and calculated surface tension values for the 790 points considered was 2.88%, which leads to establish that the model, together with the calculation scheme here proposed are accurate to reproduce the concentration and temperature dependence of the surface tension. (c) 2006 Elsevier B.V. All rights reserved.