The phase behavior of mixtures of bisphenol A polycarbonate and the zinc salts of lightly sulfonated polystyrene ionomers (Zn-SPS) was studied as a function of the sulfonation level and the molecular weight of the ionomer. This system exhibits upper critical solution temperature (UCST) phase behavior and miscibility is attributed to intramolecular repulsion between the ionic and non-ionic groups of the ionomer. The cloud point temperature decreased with increasing sulfonation level of the ionomer over the range of 8.7–13.7 mol.%, and it inscreased with increasing molecular weight of Zn-SPS for a fixed sulfonation level. Fourier transform infrared spectroscopy revealed that no specific interactions involving either the carbonate carbonyl group or the metal sulfonate group occurred in the blends. The composition dependence of the glass transition of the miscible Zn-SPS/PC blends also gave no indication that specific interactions occurred in this system.