The dynamics of vesicle formation and growth following the mixing of cationic cetyltrimethylammonium. bromide (CTAB) and anionic sodium octyl sulfate (SOS) surfactant solutions were determined using static and dynamic light scattering. The results are well-described by a two-stage model in which initial nonequilibrium vesicles are generated rapidly following the mixing of the surfactant solutions and the subsequent vesicle growth is by slower vesicle fusion. The formation of the initial vesicles is based on a balance between the unfavorable edge energy of disklike aggregates and the bending energy required to form spherical structures. The sizes of the initial vesicles correlated well with the equilibrium sizes determined after 2 months, in accordance with the model predictions. The growth of both the disklike bilayers (bilayer-bilayer fusion) before their transition to the initial vesicles and of the nonequilibrium vesicles (vesicle-vesicle coalescence) was modeled using Smoluchowski's theory for colloid coagulation. Estimates of the coalescence rate constants showed that the fusion of bilayer disks is at least an order of magnitude faster than the coalescence of nonequilibrium vesicles, with rate constants of similar to200 and similar to5-10 (Ms)(-1), respectively.