Surfactant solutions in practical applications usually are mixtures of ionic and nonionic surfactants. Because of synergistic effects, the solubilization of hydrophobic compounds can be enhanced while decreasing the needed amount of surfactant at the same time. In this work, the influence of the composition of Brij 35/CTAB and Brij 35/SDS mixed micelles on the partition coefficient log D-MW of various acids and bases over the entire pH range was investigated. Two experimental methods (MLC, micellar liquid chromatography; MEUF, micellar enhanced ultrafiltration) are evaluated for the determination of partition coefficients in mixed-micelle systems. Although MLC stands out because of its automation and easy handling, MEUF is applicable to a broader log D-MW range. It is shown that the partitioning can be influenced dramatically by the two investigated parameters. By adjusting the pH value and the composition of the micelles, we can tailor the partition behavior of solutes for virtually any application. The thermodynamic model COSMO-RS gives valuable predictions of the partition coefficients if the composition of the micelle is available. Different approaches for the description of the micellar composition are evaluated in this work. On the basis of the cmc value of the single surfactants and the mixture only, it is shown that the regular solution approximation gives reasonable micellar compositions. The partition coefficients between water and the mixed micelles are predicted with the COSMO-RS model, in good agreement with the experimental data. Moreover, the micellar composition can be evaluated by fitting the prediction to the experimentally determined partition coefficients.