Vesicles prepared in water from a series of diblock copolymers and termed “polymersomes” are physically characterized. With increasing molecular weight M̄n, the hydrophobic core thickness d for the self-assembled bilayers of polyethyleneoxide polybutadiene (PEO-PBD) increases up to ∼ 20 nm considerably greater than any previously studied lipid system. The mechanical responses of these membranes, specifically, the area elastic modulus Ka and maximal areal strain αc are measured by micromanipulation. As expected for interface-dominated elasticity, Ka (≃100 pN/nm) is found to be independent of M̄n. Related mean-field ideas also predict a limiting value for αc which is universal and about 10-fold above that typical of lipids. Experiments indeed show αc generally increases with M̄n, coming close to the theoretical limit before stress relaxation is opposed by what might be chain entanglements at the highest M̄n. The results highlight the interfacial limits of self-assemblies at the nano-scale. To whom correspondence should be addressed: School of Engineering and Applied Science, 112 Towne Bldg., University of Pennsylvania, Philadelphia PA 19104. Phone: 215-898-4809, Fax: 215-573-6334, E-mail: discher@seas.upenn.edu