Mixed micellization and surface properties of binary mixtures of cationic gemini surfactant butanediyl-alpha,omega-bis(dimethylcetylammonium bromide) (G4, 16-4-16) with conventional surfactants cetylpyriclinium chloride (CPC) sodium bis(2-ethylhexyl)sulfosuccinate (AOT), and polyoxyethylene 10 cetyl ether (Brij56) have been studied using conductometric and tensiometric methods. To explain and compare the results theoretical models of Rubingh, Rosen, Clint, and Maeda have been used to obtain the interaction parameter, minimum area per molecule, surface excess, mixed micelle composition, free energies of micellization and adsorption, and activity coefficients. The activity coefficients and experimental critical micelle concentration (cmc) values are less than unity indicating synergism in micelles as well as at interface. Also, expansion of the minimum area per molecule was observed in the binary systems supported by low values of packing parameter. (C) 2009 Elsevier Inc. All rights reserved.