The domain sizes and morphological boundaries in diblock copolymers have been predicted to be influenced by changes in the molecular weight distribution of one or both of the blocks. To systematically explore these effects, we prepared several sets of poly(ethylene-alt-propylene)-b-poly(DL-lactide) diblock copolymers with controlled molecular weights, compositions, and polydispersity indices. The polydispersity of the polylactide block was controlled by taking advantage of the equilibrium nature of the ring-opening polymerization of lactide. Small-angle X-ray scattering was used to evaluate the influence of block copolymer polydispersity on the equilibrium domain spacing and resultant ordered state morphology. We found that the domain spacing increased with increasing polydispersity and demonstrated that an increase in polydispersity at constant polylactide composition can result in a change in morphology for compositionally asymmetric diblock copolymers. More specifically, when the polylactide block is the minority component, a polydispersity increase in that block results in a change of morphology to one with larger mean interfacial curvature. Conversely, an increase in the polydispersity of the polylactide segment in a block copolymer with a majority of polylactide drives transitions to structures with smaller mean interfacial curvature. The experimental results were corroborated by self-consistent mean-field theory.