The temperature dependence of the microstructure in a polyurethane cationomer, with an ion content of 0.47 mequiv/g, is investigated using small-angle X-ray scattering (SAXS) studies and dynamic viscoelastic measurements. Both techniques indicate considerable microphase mixing occurs in the melt, with the SAXS invariant and dynamic storage modulus decreasing abruptly near the onset of melt flow at approximately 85 degrees C. Although the trends are qualitatively similar to those observed at the microphase separation transition (MST) in conventional polyurethanes, the presence of a MST cannot be stated conclusively since some level of the heterogeneous lamellar morphology persists to the highest accessible experimental temperature of 150 degrees C.