The miscibility behavior in blends of monodisperse poly(styrene) (PS) of varying molecular weight and polydisperse styrene/acrylonitrile copolymers (SAN) of varying AN contents was studied at three different temperatures: 120, 200, and 250 degreesC. An accurate quantitative evaluation of the binary interaction energy density between S and AN, B-S/AN, was made at each temperature by analyzing the miscibility data using the copolymer/critical molecular weight method. The values of B-S/AN determined by this powerful method show that the Flory-Huggins based interaction energy B-S/AN decreases as temperature increases. An equation-of-state analysis using the Sanchez-Lacombe lattice fluid theory was performed to interpret the temperature dependence of B-S/AN. An empirical scheme was introduced to account for the temperature dependence of the Sanchez-Lacombe characteristic parameters for PS. With this modification the temperature dependence of the experimental B-S/AN is well described by the Sanchez-Lacombe lattice fluid theory using a constant bare interaction energy DeltaP*(S/AN).