The phase behavior, microstructure, and emulsification of polyoxyethylene (20) sorbitan monooleate (Tween 80), water, and d-limonene (LM) or perfluoromethyldecalin (PFMD) has been studied by small-angle X-ray scattering and polarizing optical microscopy. In the Tween 80/water binary system, a micellar solution (L-1), a hexagonal (H-1) phase, and a water-swellable isotropic surfactant liquid (L-2) phase are successively formed at 25 degrees C. LM can be solubilized into all of the phases formed by Tween 80/water mixtures, whereas no solubilization of PFMD occurs. The L-2 phase was found by small-angle neutron scattering to be bicontinuous with low interfacial curvature. Added water swells and amplifies the pre-existing amphiphilic structure. The stability of oil-in-H-1 complex emulsions is found to be sensitive to changes in structure that accompany solubilization.