Abstract: Phase diagrams of water-SDS-n-butanol-toluene system with various surfactant (SDS) to cosurfactant (n-butanol) ratios are presented. The diagrams indicate that microemulsions can only be formed with a certain Combination of SDS and n-bulanol. A stretch of microemulsion domain changing continuously from direct micelles to bicontinuous structures and to reverse micelles exists when the surfactant to cosurfactant ratio is comparatively low, while the direct micelle area is separated from the reverse micelle area by a mesomorphic medium when the surfactant to cosurfactant ratio is comparatively high. Part of the tie lines in the phase separation region is also presented. The SDS content of the water-rich phase is generally higher than that of the oil-rich phase.