The cloud temperature of 2 wt% C12EO8 aqueous solutions decreases upon addition of sarcosinate-lauroyl isopropyl (SLIP), 1-dodecanol, and nt-xylene, whereas it increases in glycerol tris(2 ethylhexanoic) ester (TEH), isopropyl myristate (IPM), and saturated hydrocarbon systems. A three-phase microemulsion is formed at equal weights of water and oil in the IPM system, but a lamellar liquid crystal (L-alpha) is present in the SLIP system at the balanced temperature. The effect of added oil on the phase transition of the hexagonal (H-1) phase was also investigated by means of SAXS study. The HI-L, transition occurs upon addition of SLIP or 1-dodecanol whereas the H-1-I-1 (discontinuous micellar cubic) phase transition takes place in TEH or IPM systems. These differences in phase behavior are attributed to the placement of solubilized oil in micelles: In the former systems, oil tends to penetrate in the surfactant palisade layer and induces the surfactant layer curvature in micelles to be less positive, while the penetration tendency is small and the opposite effect on the curvature is induced upon addition Of the latter oils.