Phase behavior of a mixed surfactant, sodium dodecyl sulfate + lipophilic poly(oxyethylene) dodecyl ether, in a brine-decane system was investigated at a constant brine/decane weight ratio equal to 1. Solubilization capability of the mixed surfactant reaches its maximum and microemulsion is formed when the surfactant is changed from hydrophilic to lipophilic in a given system. In the present system, lamellar liquid crystal(LC) intrudes in the single-microemulsion region, and three-phase microemulsions are not formed. The mixing fraction of nonionic surfactant in the total surfactant in the midst of the LC present region increases with increasing oil content due to the high solubility of nonionic surfactant in oil. The partition of nonionic surfactant molecules between the oil and the bilayer in the LC phase is analyzed by using the geometrical relationship of the phase equilibria in the phase diagrams, taking into account the solubility. The monomeric solubility of nonionic surfactant in oil is much less than that of an ordinary cosurfactant like hexanol, and the mixing fraction of nonionic surfactant in the bilayer decreases with increasing salinity. The interlayer spacing of the midlamellar liquid crystal between the two microemulsion regions was measured by small-angle X-ray scattering. The average effective cross-sectional area per surfactant is about 0.37 nm(2) and is unchanged upon dilution. It is considered that there is a strong attractive interaction between the ionic group and the nonionic hydrophilic moiety of surfactants.