Abstract A three-phase region consisted of a water (W), a surfactant (D) and an oil (O) phases was found in a sodium taurocholate (NaTC)/brine/co-surfactant (monoglyceride or its model compound)/oil (triglyceride) system. A required concentration of cosurfactant to produce the three-phase region was decreased with increasing salinity. The three-phase region becomes narrower and finally vanishes with decreasing salinity at constant temperature or with the rise in temperature at constant salinity in a NaTC/brine/glycerol mono (2-ethylhexyl) ether/glycerol tris (2-ethylhexanoic) ester system. Since the critical end points of O–D and D–W phases approach and coincide in the system, the three-phase region disappears at a tricritical point at which three coexisting liquid phases become identical. According to the phase rule, there is a line of tricritical points in the space of T and compositions in this five-component system and it moves toward higher temperatures with the increase in salinity. Therefore, the three-phase region exists at temperatures below the tricritical temperature at constant salinity.