A new model for describing the surface tension of binary liquid mixtures as a function of the bulk composition over the whole concentration range is presented. We first derive an equation relating surface and bulk volume fractions that generalizes the Langmuir isotherm so as to cover the entire range of concentrations. By combining this isotherm with a new mixing rule for nonideal solutions, we obtain an equation with two adjustable parameters, one measuring the lyophobicity of one component and the other accounting for the effect of molecular interactions. The model provides an excellent description of surface tension data for a wide variety of solutions with pi (0) = sigma (A) - sigma (B) values ranging from 2.2 to 51.0 mN/m.