Poly(ethylene terephthalate) (PET) was blended with polystyrene (PS) and poly(styrene-co-maleic anhydride) (PScMA). The two blend systems were immiscible based on the glass transition temperatures measured by differential scanning calorimetry (DSC). In the case of 70 and 90% PScMA blends, the reaction between ester groups of PET and maleic anhydride (MAH) sites on PScMA occurred during melt mixing at 280 degrees C for 30 min. The reaction was confirmed indirectly by thermal data, from the increase in the complex viscosity and from the solubility difference between the blends. The particle size in PET/PScMA reactive blends was smaller than that of the non-reactive blends. The mechanical properties such as tensile strength and modulus were not affected by the reaction. (C) 2000 Elsevier Science Ltd. All rights reserved.