Microphase-separated block copolymer materials have a wide array of potential applications ranging from nanoscale lithography to energy storage. Our understanding of the factors that govern the morphology of these systems is based on comparisons between theory and experiment. The theories generally assume that the chains are perfectly monodisperse; however, typical experimental copolymer preparations have polydispersity indices (PDIs) ranging from 1.01 to 1.10. In contrast, we present a systematic study of the relationship between chemical structure and morphology in the solid state using peptoid diblock copolymers with PDIs of ≤1.00013. A series of comb-like peptoid block copolymers, poly(N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine)-block-poly(N-(2- ethylhexyl)glycine) (pNte-b-pNeh), were obtained by solid-phase synthesis. The number of monomers per chain was held fixed at 36, while the volume fraction of the Nte block (αNte) was varied from 0.11 to 0.65. The experimentally determined order-disorder transition temperature exhibited a maximum at αNte= 0.24, not αNte= 0.5 as expected from theory. All of the ordered phases had a lamellar morphology, even in the case of αNte= 0.11. Our results are in qualitative disagreement with all known theories of microphase separation in block copolymers. This raises new questions about the intertwined roles of monomer architecture and polydispersity in the phase behavior of diblock copolymers. © 2013 American Chemical Society.