Polydispersity-driven macrophase separation has been introduced into the poly(ethylene oxide) (PEO) block of poly(isoprene-b-styrene-b-ethylene oxide) (ISO) triblock polymers to stabilize the core-shell gyroid (CSG) network mesostructure at a composition where it is not stable for the monodisperse analogue. A hydroxyl-terminated IS diblock (IS-OH) was synthesized by sequential anionic polymerization in cyclohexane using secbutyllithium as the initiator. The living chains were end-capped with ethylene oxide and terminated with degassed methanol. Five ethylene oxide polymerizations were initiated from the isolated IS-OH diblock using an established protocol. Synchrotron small-angle X-ray scattering (SAXS) experiments were also conducted. The increased polydispersity in the PEO block increases the entropic penalty incurred by the PEO chains as they stretch to accommodate the CSG mesostructure.