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 (f(PI) = f(PS)) and varying amounts of PEO, 0 less than or equal to f(PEO) less than or equal to (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.