Blends of poly(ethylene terephthalate)/poly(ethylene 2,6-naphthalene dicarboxylate) (PET/PEN) have exhibited properties that are of commercial interest to the packaging industry. Melt processing of PET with PEN results in transesterification reactions. The blend properties are controlled by the kinetics of these reactions. The chemical kinetics equations have been modified to predict theoretical processing temperatures required for different blend compositions to achieve critical levels of transesterification. These values are found to be in close agreement with the experimentally observed values obtained for blends processed in a twin-screw extruder. The critical transesterification temperature is dependent on the equilibrium melting point of the miscible blend and the nature of the PET and PEN resins used for preparing the blend. (C) 2003 Wiley Periodicals, Inc.