A thermodynamic model is derived for the description of the adsorption behavior of mixed surfactant systems. The model requires the surface tensions of the single-surfactant systems or the adsorption isotherms to construct the isotherm of the mixture. The approach does not assume extra interaction parameters between the different compounds. The model is tested with a number of literature data, such as mixed sodium alkyl sulfates, nonionic surfactant mixtures, and an anionic-nonionic mixture. The agreement between experimental data and the theoretical calculations is excellent.