In this study, compatibilizing effects of maleic anhydride-styrene-vinyl acetate (MAStVA) with various terpolymer ratios on immiscible blends of poly(vinyl chloride) (PVC) and polystyrene (PS) blends were investigated by viscometry method, thermo-mechanical analysis (TMA) and infrared spectroscopy (FTIR). The viscosimetric measurements were carried out for PVC, PS, a (70:30) blend of PVC/PS and a blend of PVC/PS/MAStVA containing 2-10% of the terpolymer in the range of concentrations (0.25-2.00g dL(-1)) where tetrahydrofuran (THF) is the solvent. The interaction parameter (Ab) was used to study the miscibility and compatibility of polymer blend in solution, obtained from the modified Krigbaum and Wall theory. TMA and FTIR measurements were also used to investigate the miscibility of this pair of polymers. In consequence of the study, it was observed that a considerable improvement was achieved in the miscibility of PVC/PS blends by adding among 4 and 10 wt.% of compatibilizer. (c) 2009 Elsevier B.V. All rights reserved.