The effect of added decane, m-xylene, or squalane on the phase behavior of the polyoxyethylene dodecyl ether (C12EOn) - water system was investigated as a function of polyoxyethylene (EO) chain length at 25 degrees C. When the surfactant is relatively lipophilic (more lipophilic than the balanced state), the type of liquid crystal is changed to the more lipophilic one upon addition of decane. The lamellar (L alpha) to lipophilic reverse hexagonal (H-2) transition takes place in the C12EO3 system. On the other hand, the hexagonal (H-1) to hydrophilic discrete cubic (I-1) liquid crystal transition occurs in the hydrophilic C12EO7 system. There are two kinds of effects of oil on the self-organizing structures. One is the "penetration effect", in which oil molecules penetrate into the surfactant palisade layer and expand the effective cross-sectional area, a(s). The other is "swelling effect", in which oil molecules are solubilized in the core of aggregates and expand the volume of aggregates. In this case, the as is almost constant. Due to the swelling effect, the H-1-I-1 phase transition takes place in the decane-C12EO7 system, whereas the L alpha-H-2 transition occurs in the decane-lipophilic C12EO3 system due to the penetration effect. This causes the opposite tendency of the change in the curvature of the surfactant layers in liquid crystals on each side of the balanced state. As a result, the phase behavior of surfactant is quickly changed from forming micelles to reverse micelles within a narrow range of the EO chain in the presence of oil. On the other hand, m-xylene tends to penetrate the surfactant palisade layer, and the H-1-L alpha transition occurs even in the hydrophilic C12EO7 system.