Isothermal miscibility maps for blends of styrene-acrylonitrile (SAN) copolymers with methyl methacrylate (MMA) copolymers containing n-alkyl acrylates (xA) have been determined at 120 degreesC. The miscibility region increases and then decreases as the size of the n-alkyl pendant group increases. Binary interaction energy densities for various monomer unit pairs were evaluated from the miscibility data by both the copolymer/critical molecular weight and the copolymer composition mapping approaches using the Flory-Huggins theory combined with the binary interaction model. Values of binary interaction energy densities obtained from both approaches, with the aid of an appropriate data fitting algorithm, agreed very well with each other. Trends in binary interaction energy densities involving the various n-alkyl acrylate units, B-MMA/xA, B-S/xA, and B-AN/xA, could be roughly predicted by the simple solubility parameter theory. Trends in the deviations from the geometric mean assumption, k(ij), were qualitatively explained in terms of monomer molecular characteristics, especially polarity, monomer size, and first ionization potential.