Symmetric poly(ethylene oxide-b-styrene-b-isoprene-b-styrene-b-ethylene oxide) (OSISO) pentablock terpolymers with narrow molecular weight distributions in all blocks were synthesized by anionic polymerization. These OSISO pentablock terpolymers have a common poly (styrene-b-isoprene-b-styrene) (SIS) core containing equal volume fractions of polyisoprene and polystyrene but having different lengths of terminal poly(ethylene oxide) (PEO) chains. Small-angle X-ray scattering, transmission electron microscopy, dynamic mechanical spectroscopy, and differential scanning calorimetry were used to identify two-domain lamellae, O-70 (the orthorhombic Fddd network), and three-domain lamellae (LAM(3)) in the OSISO materials; these morphologies were previously identified in poly(isoprene-b-styrene-b-ethylene oxide) (ISO) triblock terpolymers with comparable compositions. Mechanical tensile testing was employed to probe the consequences of adding different lengths of semicrystal line PEO to the ends of intrinsically tough SIS triblock. An OSISO sample with the LAM(3) mesostructure fractured in a brittle fashion at a strain of 0.06 while an OSISO containing the O-70 network, in contrast, had a strain at failure of 1.3, even though the crystallinity of the terminal PEO blocks was above the brittle threshold established in other multiblock materials. This improved toughness is attributed to the combined effects of a triply continuous morphology and an intrinsically tough SIS core.