The mixed micelle formation in basic aqueous solutions between an anionic gemini amino acid-based surfactant derived from cysteine (C(8)Cys)(2) and the bile salts sodium cholate (NaC) and sodium deoxycholate (NaDC) has been studied by conductivity, and the results have been compared with the ones obtained for binary solutions of (C(8)Cys)(2) with the conventional anionic surfactant sodium dodecyl sulfate (SDS). A nonideal mixing pattern was observed for the mixed systems, and a striking behavior was found for the gemini surfactant-bile salt mixtures, with gemini-bile salt interactions changing from synergistic to antagonistic with increasing gemini surfactant composition, whereas for the (C(8)Cys)(2)/SDS mixed system, synergism was attained over all the molar fraction range studied. Regular solution theory (RST) was used to analyze the gemini surfactant-anionic surfactant binary mixtures, and the interaction parameter (beta) has been evaluated, as well as mixed micelle composition.