In aqueous solution at a pH of about 6.5, the micellization of binary mixtures constituted by sodium n-dodecyl diamine sulfonate (C12AS) and three octylphenol polyoxyethylene ether (OP-n) with different numbers of methylene glycol ether (n), including OP-4, OP-7, and OP-10, was examined by the UV-vis spectroscopy method. The critical micelle concentration (cmc) values were determined by the UV-vis spectroscopy method and the tensiometric method. The experimental data were analyzed according to various mixing thermodynamic models within the framework of the pseudophase separation model. For all mixed systems, the negative deviation of experimental cmc values from the ideal values calculated from the Clint equation was found, and negative values for the interaction parameter were obtained, suggesting nonideal mixing of two surfactants in aqueous solution and a synergistic effect between surfactants. Among the C12AS/OP-n mixed systems, the stronger hydrophilicity of nonionic surfactant results in an increase of values of minimum cmc or optimum molar fraction, where the maximum synergism between surfactants occurs, and the C12AS/OP-7 system presents the highest degree of synergism between surfactants in aqueous solution, resulting from their suitable molecule structure. The electrostatic interaction between head groups of C12AS and the effects of steric character of head groups of both nonionic surfactant and C12AS together affect the mixed micelle formation. Thermodynamic data show that the micellization for all the binary mixed systems is spontaneous and is an enthalpy- and entropy-driven process, and the C12AS/OP-7 system has the greatest stability of micellization.