The underpinnings of microphase separation in symmetric poly(styrenesulfonate-block-methylbutylene) (PSS-PMB) copolymer melts were examined by Monte Carlo lattice simulations. The main challenge is understanding the effect of ion pairs in the PSS block on thermodynamics. We assume that experimentally determined Flory-Huggins interaction parameters are adequate for describing intermonomer interactions. Our model does not account for either electrostatic or dipolar interactions. This enables comparisons between simulated and experimentally observed microphases reported by Park and Balsara [Macromolecules 2008, 41, 3678] without resorting to any adjustable parameters. The PSS block in both experiments and theory is partially sulfonated. We quantified the effect of sequence distribution on phase behavior by using alternating and blocky PSS chains in the simulations. Depending on temperature and sequence distribution, simulations show performed lamellae, gyroid, and hexagonally packed cylinders in addition to the lamellar phase found in simple symmetric block copolymers that do not contain ions. This is driven by extremely repulsive interactions between styrenesulfonate monomers and the uncharged species in the melts. The symmetry of the microphases and the locations of the order-disorder and order-order phase transitions are in qualitative agreement with experimental observations.