Interfacial tensions of aqueous two-phase systems formed by mixtures of cationic and anionic surfactants were measured by the spinning drop method. The effect of the molar ratio of cationic surfactants to anionic surfactants (C +/C -) and the alkyl chain symmetry on the interfacial tensions were investigated. It was showed that values of the interfacial tensions of the aqueous two-phase systems were in the scale of ultralow interfacial tensions. Two types of γ～C +/C - curves were observed, in which DoTEAB/SDS and OTEAB/SDS systems were the typical representatives (DoTEAB, OTEAB and SDS are the abbreviations for dodecyltriethylammonium bromide, octyltriethylammonium bromide and sodium dodecylsulfate, respectively). The two types of curves were explained in terms of the change of the mixed micelles composition and symmetry of hydrophobic chains of cationic and anionic surfactants with molar ratio.
