Microemulsions are formed over a wide range of temperatures in brine/sodium dodecyl sulfate/polyoxyethylene dodecyl ether (C12EO2 Or C12EO3)/decane systems. In a dilute region, lamellar liquid crystal is present instead of bicontinuous microemulsion at the composition at which the HLB of the mixed surfactant is optimum in the given system. The monomeric solubilities of C12EO2 or C12EO3 in oil and the mixing fraction of C12EO2 Or C12EO3 in the mixed surfactant layer were determined using the geometrical relation of three-phase tie triangle containing the liquid crystal phase in the composition tetrahedron and were compared at 35 and 60 degrees C. The change in the HLB composition with temperature is well described by increasing the monomeric solubility and decreasing the mixing fraction of nonionic surfactant in the surfactant layer. As a result, temperature-insensitive microemulsions are formed in the SDS-C12EO3 system,