The micellization properties of binary mixtures of sodium n-dodecyl diamine sulfonate (C12AS) and octylphenol polyoxyethylene ether (7), OP-7, in aqueous solution at the pH of 3.5, 6.0, and 8.5 have been characterized mainly employing both the UV-vis spectroscopy method, using pyrene as a probe, and the tensiometric method. The interaction parameters between two surfactants, the activity coefficients, the molar fractions of components in mixed micelles, and thermodynamic parameters obtained using some theoretical treatments including the regular solution theory (RST), the phase separation model, the Rubingh model, Maeda’s treatment, and so on, were evaluated and the effect of pH on the above parameters was also discussed. Reduction of the mixed critical micelle concentration and negative values of interaction parameter indicate synergistic effect between two surfactants for all mixed systems. The charge density and steric effect of head group of C12AS may play a vital role on the formation of mixed micelle, and the values of pH in aqueous solution can influence the ionic form of C12AS and this steric effect, thus resulting in different interacting behaviors between two surfactants. Thermodynamic data show that the process of micellization for all mixed systems is spontaneous, the stability of mixed micelle is mainly dependent on the components in bulk solution, and the more stable mixed micelle can form in neutral or weak acidic solution.