The solubilization of triglycerides [1,2,3-tributanoylglycerol (TBG) and 1,2,3-trihexanoylglycerol (THG)l in water/octa(oxyethylene) dodecyl ether (C12EO8) systems has been investigated. Oil-induced changes in the structure of liquid crystals in water/C12EO8 system have been studied by optical observation and small-angle X-ray scattering (SAXS) measurements. In the water/C12EO8/oil systems, solubilization of THG and TBG induces a transition between H-1 (hexagonal) and L-alpha (lamellar) liquid crystals at high C12EO8 concentrations, whereas at low surfactant concentrations a H-1-I-1 (discontinuous micellar cubic phase) transition occurs. This anomalous behavior is attributed to the partitioning of solubilized oil in the micelles. At low surfactant concentrations THG is mainly solubilized into the hydrophobic cores of the surfactant micelles, indicating high swelling or low penetration tendency, resulting in a steep increase in the radius of the aggregates (r(H)), thereby inducing a rod-sphere transition. At high surfactant concentrations, THG is not mainly solubilized into the core but distributed between the palisade layer and the core of the aggregates. The TBG is considerably solubilized into the surfactant palisade layer, indicating a high penetration tendency, resulting in an increase in the effective cross-sectional area per surfactant molecule, a(s). The thermal stability of the I-1 phase increases with the solubilization of THG into the aggregate cores. The percentage deviation of the experimental interlayer spacings (P-d) from complete swelling was also evaluated for different triglycerides in the H-1 and L-alpha phases or different surfactant concentrations. It is found that the penetration tendency of triglycerides could be used as a tuning parameter for I-1 phase formation depending on the surfactant concentration and the molecular weight of the oil. (C) 2008 Elsevier Inc. All rights reserved.