The microphase separation of diblock copolymers synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, containing one monodisperse block (poly(methyl methacrylate), PMMA) and one polydisperse block (poly(butyl acrylate), PBA), was investigated in thin films (<100 nm). The formation of ordered microphase-separated domains was observed by atomic force microscopy (AFM) and resulted in four morphologies, depending on composition and film thickness: parallel lamellae, hexagonally packed perforated lamellae (PL), parallel cylinders (C∥), and hexagonally packed spheres, and in C∥to-PL-to-C∥ transitions. Polydispersity of the PBA block shifts the phase boundaries toward higher PBA volume fraction values with respect to those expected for monodisperse block copolymers and stabilizes the perforated lamella morphology. Neutron reflectivity data confirmed that lamellae parallel to the substrate form at a very low PBA volume fraction, fPBA = 0.23. Polydispersity of the PBA block also has the effect of stabilizing each microphase domain over a film thickness regime larger than expected for monodisperse blocks. For the first time RAFT-polymerized block copolymers are shown to microphase separate with high reproducibility and with excellent degree of order, hence proving to be ideal systems to test the effect of polydispersity on microphase separation.