The effect of polydispersity on the self-assembly of block copolymer aggregates in solution was studied under two sets of conditions. A series of polystyrene-block-poly(acrylic acid) copolymers of an identical polystyrene length of 310 units but of varying degrees of polymerization of the poly(acrylic acid) (PAA) was synthesized. Mixtures of the copolymers were made to artificially broaden the molecular weight distribution of the PAA at a constant number average of 28, in the polydispersity index (PI) range of 1.1-2.1. The samples were dissolved in two different solvent systems (dioxane and a mixture of tetrahydrofuran(THF)/N,N-dimethyl formamide(DMF)), and self-assembly was induced by the slow addition of water. The samples were quenched at a predetermined water content. Transmission electron microscopy and dynamic light scattering were used to measure sizes and size distributions of the aggregates. At a low PAA polydispersity index (PAA PI similar to 1.1), in the THF/DMF system, large polydisperse vesicles of a diameter of 270 +/- 220 mu were seen. Generally the size of the vesicles decreased with increased PAA polydispersity. For example, in the THF/DMF system, at a PAA PI of 1.8, the average vesicle size was 80 +/- 20 nm. The trend is explained qualitatively by the length segregation of the PAA chains (large chains segregate to the outer surface while smaller chains prefer the inner surface of the vesicle) and the ratio of short chains to long chains in the mixtures. The presence of only vesicles in mixtures containing polymers that do not form vesicles by themselves, but rather spheres and large compound micelles, suggests that there is no significant segregation of chains between aggregates, but only segregation of chains within individual vesicular aggregates.