The micellization behavior of binary mixtures of sodium n-dodecyl diamine sulfonate (C12AS) and octylphenol polyoxyethylene ether (10), OP-10, in aqueous solutions at pH 3.5, 6.0, and 8.5 was investigated, and the values of mixed critical micelle concentration (C-M) were determined by both UV-vis spectroscopy using pyrene as a probe and by tensiometry. Within the framework of the pseudophase separation model, four thermodynamic models were adopted to describe the micellization parameters including the interaction parameters ((12)) between two surfactants, the components in real and ideal mixed micelles, the activity coefficients in mixed micelles, the thermodynamic parameters, and the thermodynamic stability. The experimental values of C-M, inferior to the ideal values, show nonideal mixing; the negative values of (12) obtained from Rubingh's, Rodenas's, and Motomura's models mean the synergistic effect. Although there exist some divergences in the micellization parameters predicted by the three treatments, similar results were obtained, indicating the validity of these models in this investigation. Based on the calculation errors, Rubingh's model is best to describe the interacting behavior between the two surfactants in this investigation. Thermodynamic parameters predicted by the three treatments show a spontaneous process of micellization and an entropic contribution in the formation of mixed micelles. Different acidities of solutions result in different stabilities of micellization. The stability in feeble acidic solution is higher than in acidic or basic solution, and the stability in basic solution is slightly superior to that in acidic solution. All the above described phenomena can be explained rationally by the electrostatic effect of head groups of C12AS, the steric effect of head group for two surfactants, the molecular structures of surfactants, the influence of added counterion, and so on. (c) 2015 Society of Chemical Industry