The phase behaviour of 50/50 blends of a series of terpolymers from styrene, maleic anhydride and acrylonitrile with styrene-acrylonitrile copolymers and styrene-maleic anhydride copolymers has been determined using the single glass transition temperature criterion. The appropriate expression for the overall blend interaction energy density according to the extended Flory-Huggins theory is examined and the general shapes of miscibility contours in composition space are deduced and presented in a novel manner. The experimental miscibility is dependent on both terpolymer and copolymer composition, and is well described by this expression using values of segmental interaction parameters available from the literature.