The influence of broad and continuous center block polydispersity on the melt-phase self-assembly of OBO triblock copolymers (O = poly(ethylene oxide) and B = poly(1,4-butadiene)) is reported for a series of samples derived from tandem chain transfer ring-opening metathesis polymerization (ROMP-CT) and anionic ring-opening polymerization (AROP). By virtue of the polymerization techniques employed in these syntheses, the midblocks exhibit polydispersity indices M_w/M_n = 1.75, whereas the end blocks have relatively narrow dispersities M_w/M_n ≤ 1.25. Using a combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) to characterize these materials and their narrow dispersity homologues derived from anionic polymerization, we demonstrate that the combination of both chain length and composition polydispersity inherent in these polydisperse triblocks shifts the composition window of stability for the lamellar mesophase. Furthermore, these studies reveal that polydispersity in the center B segments of these OBO triblocks results in large lamellar domain spacing increases, enables stable coexistence of two morphologies in a single sample, and frustrates lattice ordering in a composition-dependent manner.