To investigate whether diblock copolymers with substantial distributions of molecular weight and composition can self-assemble into highly regular microdomain structures, we examine the morphologies of polystyrene-b-poly(acrylic acid), diblock copolymers (PS-PAA) synthesized by controlled radical polymerization in emulsion. Despite diblock polydispersity indices of ca. 2, solvent-cast films of these diblocks, as well as their blends with PS homopolymer of low molecular weight, exhibit well-ordered lamellar or hexagonally packed cylindrical morphologies in the midrange of composition, as shown by small-angle X-ray scattering and transmission electron microscopy. In several specimens, well-ordered regions of lamellae and cylinders appear to coexist. Films of very asymmetric diblocks or blends exhibit spherical microdomains without lattice order.