The behavior of binary mixtures of aqueous solutions of surfactants has been analyzed in the light of (a) Rubingh’s approach based on regular solution theory, (b) the method of excess thermodynamic quantities of Motomura et al., and (c) the molecular thermodynamic theory of Sarmoria et al. and Puvvada et al. The binary combinations of anionic-nonionic, anionic-cationic, anionic-anionic, cationic-cationic, and nonionic-nonionic surfactant systems including a number of bile salts and several synthetic ionic and nonionic surfactants have been tested. The theoretical approaches are found to be most successful in describing the mixed micellar properties of anionic-nonionic surfactant solutions. The anionic-cationic combinations of bile salts and cetyltrimethylammonium bromide exhibit striking behaviors. The scope and limitations of the treatments in describing and also predicting the behaviors of different types of mixed micellar entities have been discussed.