A study of the phase and theological behavior of sucrose hexadecanoate (C16SE)/cosurfactan/water systems in the presence of solubilized oil, using complementary techniques such as dynamic light scattering and small angle X-ray scattering, is presented. Viscoelastic wormlike micellar solutions are found when a nonionic lipophilic cosurfactant is added to C16SE aqueous systems. Contrary to previous reports, the effect of oil solubilization on these wormlike micelles is not unique and depends on several factors. Linear alkyl chain oils that tend, to solubilize in the micellar core have a disrupting effect, decreasing the relaxation time and the viscosity of the systems. This effect is larger as the molecular volume of oil increases and as the solubility of the cosurfactant in oil increases. On the other hand, oils that. penetrate in the palisade layer, such as p-xylene, induce micellar growth and have a thickening effect at a given micellar composition. Thermodynamic considerations are used to explain the experimental results. (c) 2005 Published by Elsevier Inc.