The intrinsic viscosities of blends of poly(vinyl chloride)/poly(ethylene-co-vinyl acetate) (PVC/EVA), poly(vinyl chloride)/poly(styrene-co-acrylonitrile) (PVC/SAN), and poly(ethylene-co-vinyl acetate)/poly(styrene-co-acrylonitrile) (EVA/SAN) have been studied in cyclohexanone as a function of blend composition. In order to predict the compatibility of polymer pairs in solution, the interaction parameter term, DELTAb, obtained from the modified Krigbaum and Wall theory, and the difference in the intrinsic viscosities of the polymer mixtures and the weight average intrinsic viscosities of the two polymer solutions taken separately are used.