Isothermal miscibility maps for blends of styrene-acrylonitrile (SAN) copolymers with methyl methacrylate (MMA) copolymers containing ethyl acrylate (EA) and n-butyl acrylate (nBA) have been determined. The miscibility region for MMA-nBA copolymers is larger than that for MMA-EA copolymers. Binary interaction energies for monomer unit pairs were calculated from critical molecular weight experiments and data from the miscibility maps using with the Flory-Huggins theory combined with the binary interaction model. Spinodal temperatures predicted from the lattice-fluid theory of Sanchez and Lacombe, using these experimental interaction energies, are similar to the experimental phase separation temperatures.