The effect of adding a Thermomyces (formerly Humicola) lanuginosa lipase (TLL) to a cubic dispersion of the binary monoolein (MO)-(H2O)-H-2 system and to vesicles formed by the ternary MO-sodium oleate (NaO)(H2O)-H-2 system at high water content has been studied. The change in lipid aggregate structure during the lipolytic process was observed by light microscopy, cryo-TEM, and dynamic light scattering, and the lipid composition was determined with HPLC. The sequence of lipid aggregate structures observed upon lipase addition to the cubic dispersion is cubic dispersion --> reversed cubic, hexagonal, and lamellar structures --> large polygonal structures --> dispersed oil droplets. The addition of lipase to MO/NaO aqueous-based vesicles did not change the morphology significantly. However, close to steady state, the vesicles changed from having smooth interfaces to irregular ones. When lipolysis reached steady state, the MO/(OA+NaO) ratio was similar in the two systems studied, although the morphology of the structures formed during lipolysis differed significantly. These differences are discussed in relation to the degree of dissociation of the fatty acid.