The mixing behavior of surfactants was investigated by using the phase separation model. For the nonideal behavior of surfactants we took into consideration the most common model of Rubingh. It was shown that this so-called regular solution theory is just an empirical approach and its applicability is not a matter of the vanishing excess entropy. The error of the interaction parameter beta of this theory was calculated and it was shown that the error is higher the more asymmetric the micellar mixing ratio, the more different the cmc values of the used components, and the more positive beta itself It was shown that mixtures of surfactants can feign a wrong cmc* value, when the component with the much higher cmc has the highest mole fraction. Mixing an anionic perfluorinated surfactant with a nonionic surfactant with nearly identical cmc values, it was shown that the one-parameter Rubingh model is insufficient. The investigated binary mixtures show S-shaped cmc* (alpha) curves. These curves can only be described by more parameters. The interaction between the different headgroups, that means the decrease of the electrostatic and steric repulsion in a mixture of an ionic and a nonionic surfactant with respect to the pure components, makes DELTAG(ex) negative for nearly all mixing ratios. The positive contribution of the chain phobicity of the hydrocarbon and perfluorinated chains to DELTAG(ex) is much smaller and only determines the sign of DELTAG(ex) at very high portions of anionic surfactant and the sign of DELTAV. The asymmetry of the DELTAG(ex) curve is due to the change of the interaction with the micellar composition x.