Adsorption isotherms for concentrated aqueous-organic solutions are presented. These are calculated from the Gibbs adsorption equation, using surface tension and activity coefficient experimental data. General considerations reveal that two maxima may be expected in the adsorption isotherm. Moreover, close to phase separation, the adsorption isotherm may show singularities. Calculations for specific examples support these general predictions. The calculated adsorption isotherms are qualitatively different from existing theoretical models due to the high concentrations involved and to the nonideality of the solutions.