Poly(ethylene terephthalate)-poly(m-xylylene adipamide) PET-MXD6 copolymers were prepared by reactive blending of equimolar PET/MXD6 blends at 285 degrees C for different times in presence of terephthalic acid (1 wt %). First, the partial hydrolysis of PET and MXD6 occurs, yielding oligomers terminated with the reactive aromatic carboxyl groups. These oligomers quickly react with ester and amide inner groups producing a PET-MXD6 copolymer that may compatibilize the initial biphasic blend. In this homogeneous environment, the aliphatic carboxyl-terminated MXD6 chains, inactive in the initial biphasic blend, may promote the exchange reactions determining the formation of a random copolymer at longer reaction time (120 min). The progress of exchange reactions, and the microstructure of the formed copolyesteramides, versus the reaction time was followed by H-1 and C-13 NMR analyses using a CDCl3/TFA-d/(CF3CO)(2)O mixture as solvent and applying appropriate mathematical models. Dyads and triads sequences were thoroughly characterized by NMR. Semicrystalline block copolymers were obtained at reaction time lower than 45 min. All PET-MXD6 copolymers show a single T-g that change as a function of the dyads molar composition in the copolymers. The measured T-g values match with those calculated by a proposed modified Fox equation that take into account the weight fraction of the four dyad components of the PET-MXD6 copolymers. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5135-5155, 2010