Blends of styrene/acrylonitrile copolymers (poly-SAN) with imidized PMMA exhibit increased thermoresistance in comparison with poly-SAN alone. The miscibility of two modified poly (methyl methacrylate) (PMMA) with poly-SAN has been evaluated with the aid of different techniques : isothermal calorimetry, through which mixing enthalpy and the Flory-Huggins chi parameter can be determined; dielectrical spectroscopy and thermally stimulated discharging currents (TSDC). The results obtained exhibit great consistency, thus supporting the usefulness of the applied methods for determining the mixing behavior of these polymer blends, which can be considered as model systems for thermoresistant ABS nowadays widely used in the automotive industry. Furthermore, the applied methods allow one to obtain useful data for the electrical application of the investigated blends.