The sequence of phases formed by amphiphilic copolymers on dilution in water has been studied for two poly(ethylene oxide)-co-poly(butylene oxide) copolymers with the same hydrophobe-hydrophile ratio but different molecular weights. The size of the amphiphile significantly affects both the formation of lyotropic phase and dispersed vesicles. The evolution from the bulk solid to lyotropic liquid shows that the low molecular weight copolymer assembles first into an inverted hexagonal structure and then lamellae. whereas the high molecular weight dissolves directly to the lamellar phase. The transition from lamellae to the sponge phase is essentially unaffected by the molecular weight of the amphiphiles. In contrast, however, the evolution of the sponge phase into vesicles is qualitatively different depending on the size of the amphiphile. The smaller amphiphile forms dispersed vesicles at quite high concentrations, whereas the high molecular weight polymer initially forms peculiar vesicular gel clusters, which eventually break up into dispersed vesicles. The amphiphile size also has a particular effect on the nature of the vesicles formed, the larger amphiphile forms uniquely unilamellar vesicles on dilution whereas the smaller amphiphiles make multilamellar vesicles while still in a lyotropic solution. Obviously, membrane undulations and membrane unbinding depend strongly on the stiffness of the amphiphilic membrane, which is dominated by the size. More flexible membranes lead to earlier unbinding and multilamellar vesicles, and as membrane flexibility is reduced unbinding occurs at lower concentrations and unilamellar vesicles result.