AbstractSynergism in mixed micelle formation and surface tension reduction efficiency and the ternary phase behavior of anionic surfactant (alcohol polyoxyethylene ether acetate containing 10 ethylene oxide group and a fatty chain of C16–18) with cationic surfactants (dodecyldimethylbenzyl ammonium chloride and lauryltrimethyl ammonium chloride) were investigated. Surface tension of the systems at different molar ratios was studied in detail and the interaction parameters of each system were calculated. The results show that both systems have lower values of critical micelle concentration (CMC) and γcmc than individual surfactants especially at equal ratio between cationic and anionic surfactants. Both systems present synergism in mixed micelle formation and surface tension reduction efficiency. The ternary phase behavior of the two systems was investigated using a polarized microscope. The micellar phase and lamellar phase were observed in both systems and the coexisting phase was only observed in the dodecyldimethylbenzyl ammonium chloride system.