Structural change in hexagonal liquid crystal (H-1) in the water/heptaoxyethylene dodecyl ether (C12EO7) system upon addition of mixed oil (m-xylene + decane) is investigated by phase study and small-angle X-ray scattering (SAXS). The surfactant layer curvature tends to be less positive or negative upon addition of the mixed oil at a high m-xyleneldecane ratio, and the H-1-L-alpha (lamellar liquid crystal) transition takes place. At a low m-xylene/decane ratio, however, the H-1-I-l transition occurs with increasing mixed oil content, where I-l is a discontinuous micellar cubic phase. An excess oil phase directly separates from the H-1 phase at a moderate m-xyleneldecane ratio. The change in the effective cross sectional area per surfactant, a(S), as a function of oil content was also measured at each mixing fraction of oils by SAXS. m-Xylene molecules tend to penetrate the surfactant palisade layer and widen a(S), whereas decane molecules tend to be solubilized in the deep core of surfactant hydrophobic moieties and the us is not largely expanded. The a(S) does not jump at each transition of H-1-I-l and H-1-L-alpha. Considering the volume of polyoxyethylene chain (EO chain), the repulsion between the EO chains may increase upon addition of decane, whereas it would decrease upon addition of m-xylene. To balance the repulsion with the attraction due to the interfacial tension between water and the lipophilic part without the big change in a(S), the phase transitions take place but the change in the surfactant layer curvature is opposite in both oil systems.