Monomer concentration changes in mixtures of a cationic surfactant, tetradecyltrimethylammonium chloride (TTAC), and a nonionic surfactant, pentadecyl ethoxylated nonylphenol (NP-15), have been studied with the help of the ultrafiltration technique. While the pseudophase separation model is valid for describing the micellization behavior of single surfactants, it was found to fail for mixed systems. In the case of TTAC and NP-15 mixed systems, both the monomer concentrations and mixed micellar composition changed continuously above the critical micelle concentration (cmc) of-the mixtures. The measured changes of monomer concentrations above the mixed cmc were different from those predicted by the regular solution theory. Two different mixed micelles, TTAC-rich and NP-15-rich micelles, are proposed to form above a concentration range which in turn depends on the ratio of the two surfactants in the mixed system.