The interchange reaction in blends of poly (ethylene terephthalate) (PET) and poly(m-xylylene adipamide) (MXD6) has been characterized in terms of changes observed in spectra obtained with a 600-MHz H-1-NMR. The selective degradation of PET components in the blends was carried out in the NMR tubes prior to evaluation. Results indicate that there is no chemical reaction between the PET and MXD6 in the absence of sodium P-toluenesulfonate catalyst. The presence of the catalyst activates the interchange reaction between these two resins. A mathematical method was applied to calculate the degree of randomness of PET-MXD6 copolymer. In addition, the reaction degree was found to be affected by exposure temperature, time, shear rate, and catalyst concentration. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3449-3461, 2009