A combination of NMR and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) techniques were suitable tools for examining the exchange reactions that occur during the melt-mixing of nylon-6 and poly(ethylene terephthalate) (Ny6/PET) blends in the presence of p-toluene sulfonic acid (TsOH) at 285 degrees'C. Some researchers believe that TsOH is an efficient catalyst for the amide-ester exchange reactions in PET/Ny6 and PET/nylon-66 blends in the molten state. Instead, we have found that TsOH is able to react in the molten state with PET, yielding PET oligomers terminated with carboxyl groups. Because the latter oligomers can quickly react with Ny6 producing a Ny6/PET copolymer, the role of TsOH in the melt-mixing process is not that of a catalyst but of a reactant. Our study allowed the structural identification of the Ny6/PET copolyesteramide produced in the exchange as a function of melt-mixing time. The results revealed the essential role of carboxyl end groups in the exchange reaction between Ny6 and PET and allowed a detailed mechanism for this reaction. (C) 2003 Wiley Periodicals, Inc.