The aqueous mixtures of cationic-anionic surfactants at relatively high concentrations (much higher than CMCs) can separate spontaneously into two immiscible aqueous phases. The formation conditions, compositions, and physicochemical properties of the aqueous two phases have been studied. This phase separation phenomenon is likely due to the coexistence of different kinds of micelles. It is shown that such systems may be taken as partitioning systems to be used in the partitioning and analysis of biomaterials, which is demonstrated by the partitioning of bovine serum albumin (BSA) in aqueous two-phase systems.