Abstract Bile salts are biologically important amphiphiles whose micellar behavior is considerably different from that of ordinary surfactants. In this work pre-micellar sodium dehydrocholate (NaDHC)–sodium deoxycholate (NaDC) mixed aqueous solutions composition is determined and their surface tension behavior is interpreted based on the information about the bulk solution composition which was obtained from the analysis of pH data. We used the pH of carboxylate soap solutions theory developed by Lucassen and generalized by Kralchevsky et al. In general it was found that the NaDHC:NaDC mixture pre-micellar solutions were mostly composed by acid soaps like complexes with different compositions and no presence of neutral soap was identified. The equilibrium solubility ( S HZ) and dissociation constant of acid ( K ˜ A ) values found for pure NaDC and NaDHC were 6.081 × 10 −7 and 0.0797, respectively, similar results were obtained from literature. Surface tension isotherms were interpreted from the pH data. It can be seen that the presence of precipitates notably affected the surface tension behavior of rich NaDHC solutions but this conduct decreased as the content of DC − ion in the mixture augmented. This fact is probably due to the formation of H-bonds between DC − and water molecules and the consequent monolayer saturation.