Abstract: By using titration and chromatograph analysis, phase diagrams of H_2O/C_(12)H_(25)-SO_4Na/n-C_4H_9OH/n-C_8H_(18) system with various ratios of surfactant (SDS) to cosurfactant (n-C_4H_9OH) were measured. The diagrams indicated that microemulsions can only be formed in a certain range of combination of SDS and n-C_4H_9OH. The microemulsion domain first expands gradually as soonas the ratio of SDS increases. The W/O type microemulsion, bicontinuous region and the O/W microemulsion can be distinguished using the electric conductance determination. The bicontinuous domain also expands with the increase of SDS content. However, when the ratio of SDS continuously increases to a very large extent, the microemulsion domain ceases to expand. On the contrary, it gradually reduces due to the diminishing of solubilities of SDS and oil mixtures in water and to the appearance of viscous liquid crystal phase. The determination of tie lines further indicated that the SDS content in water-rich microemulsion phases is generally higher than that in the oil-rich phases.