We present characterization data for linear poly(isoprene-b-styrene-b-ethylene oxide) (PI-PS-PEO) triblock copolymers containing equal volume fractions of PI and PS (fPI = fPS) and varying amounts of PEO, 0 ≤ fPEO ≤ 1/3. Detailed characterization by TEM, SAXS, dynamic mechanical spectroscopy, static birefringence, and DSC indicates three distinct morphologies: two-domain lamellae, a three-domain noncubic triply periodic network structure, and three-domain lamellae with increasing PEO content. This phase behavior is in striking contrast with the homologous PS-PI-PEO triblock phase diagram, reported earlier, which contains four different morphologies between the limiting two- and three-domain lamellae phases. Preliminary analysis suggests the noncubic network structure contains elementary units of 3-fold coordination, yet fundamentally distinct from the cubic core-shell gyroid topology previously demonstrated in PS-PI-PEO. This tentatively assigned model is a structural hybrid of the gyroid and perforated lamellar morphologies. These results provide quantitative evidence regarding the effects of block sequencing on the phase behavior of ABC triblock copolymers near the order-disorder transition.