Amphiphilic molecules spontaneously self-assemble in solution to form a variety of aggregates. The understanding of the equilibrium properties of these aggregates, such as their shape and size, has made significant progress. However, only little is known about the existence of non-equilibrium or metastable states which form during structural transitions and the kinetics of their formation. Aqueous mixtures of lecithin and bile salt exhibit a transition from worm-like mixed micelles to vesicles, which spontaneously occurs upon dilution. This transition is studied using time-resolved light and small-angle neutron scattering. It is demonstrated that the temporal evolution of the aggregate structures can be followed and detailed information even on molecular length scales can be obtained.