The aqueous two-phase regions and the relevant properties of water/sodium dodecyl sulfonate (SDS)/catyl trimethyl amino bromine (CTAB) system have been determined. It is shown that there are two aqueous two-phase in two regions with excess SDS or CTAB. The one with excess SDS has cloud point. Obvious phenomena of polarized light can be detected in the upper phase. Another aqueous two-phase region with excess CTAB has the property of Krafft point and the phenomena of polarized light isn't remarkable in the upper phase. The freezing etch TEM photograph showed that the structure of upper phase is lamellar and the lower phase is sphere-like. The volume ratio of two phases is sensitive to minor variations of the ratio between anionic surfactant and cationic surfactant.

Key words: SURFACTANTS, PHASE DIAGRAM, MICRO-STRUCTURE, LAURYL SODIUM SULFATE