ABC triblock copolymers are capable of self-assembling into triply-periodic network structures with a high degree of internal interfacial area and tailored chemical and mechanical properties, making them perfect candidates for nanoscale devices. Our present work focuses on the bulk morphological behavior of the poly[isoprene-block-styrene-block-(methyl methacrylate)] (ISM) system near the styrene-rich network phase window. Lamellar (LAM(3)), alternating gyroid (Q(214)) and disordered melt (DIS) morphologies were found. Our experimental phase behavior is compared to the self-consistent-field theory diagram of a nonfrustrated ABC model system by Tyler et al. The behavior of our ISM system differs slightly from the model ABC system due to small discrepancies in conformational asymmetries and interaction parameters between the respective systems, which highlights the sensitivity of self-assembly in triblock copolymers.