Isothermal phase boundaries for three separate copolymer blend systems, involving the monomers itaconic anhydride, methyl methacrylate, styrene and acrylonitrile, have been established as a function of copolymer composition. Analysis of these in terms of the Flory-Huggins approach and the isothermal blend interaction energy density parameter B(blend) yields an estimate of the segmental interaction of itaconic anhydride with each of the other comonomers.