Copolymers of methyl methacrylate (MMA) with 2-hydroxyethyl methacrylate (HEMA) and 4-methacryloxyethyl trimellitic anhydride (4META) were synthesized. The miscibility regions, for blends of these copolymers with styrene-acrylonitrile (SAN) copolymers were determined. The miscibility region for MMA-HEMA copolymers is larger than that for MMA-4META copolymers. Interaction energies for monomer unit pairs were calculated from the isothermal miscibility maps using the Flory-Huggins theory combined with the binary interaction model. The experimental phase separation temperatures were found to be similar to the spinodal temperatures predicted from the lattice-fluid theory of Sanchez and Lacombe using these interaction energies. (C) 2000 Elsevier Science Ltd. All rights reserved.