Two series of polyether polyurethane anionomers based on 4, 4-diphenylmethane diisocyanate (MDI) and polytetra-methylene oxide (PTMO) were synthesized and their properties investigated. Ionization proceeded via a bimolecular nucleophilic displacement reaction with sodium hydride, followed by a ring-opening reaction with γ-propane sultone. The lower hard segment content (20 wt% MDI) control material exhibited an initial one-phase morphology that underwent phase separation upon ionization, with subsequent development of hard segment ordering. These changes in morphology were accompanied by dramatic improvements in mechanical properties. The 38 wt% MDI control material possessed a two-phase morphology and well-developed hard segment ordering. Ionization disrupted the hard segment ordering and at low levels decreased the degree of phase separation. Further ionization led to reattainment of the original degree of phase separation and a marked improvement in mechanical strength. The differences in properties of the two series of materials are discussed and compared with properties of other polyurethane ionomers reported in the literature.