The effect of salt on the phase behavior of aqueous two-phase system (ATPS) formed by sodium dodecyl sulphate (SDS) and cetyl trimethyl ammonium bromide (CTAB) aqueous mixture has been studied. In the ternary phase diagram of 0.2 mol•L^－1 SDS/0.2 mol•L^－1 CTAB/H₂O systems, there are two ATPS regions, which are called ATPS_a region (containing more SDS) and ATPS_c region (containing more CTAB). When salt was added to the system, ATPS_a region and ATPS_c region moved towards SDS and CTAB respectively, and the width of each ATPS region was enlarged. The effect of salt would be saturated if the concentration of salt in system attain to certain value. It was easier to attain the saturated state in ATPS_c than in ATPS_a. The larger the radius of salt ion in the counter ion diffusion layer is, the stronger the effect on the position and the width of ATPS regions is. But the effect would be restricted by the opposite ion of salt.

Key words: aqueous two-phase system (ATPS), cationic surfactant, anionic surfactant, liquid crystal, conductivity