A combination of NMR and MALDI was found to be a suitable tool to study the exchange reactions that occur during the melt mixing of Nylon-6 and poly(butylene terephthalate) (Ny6/PBT) blends. The results reveal the essential role of carboxyl. end groups in the exchange reaction, and allow drawing a detailed mechanism for this reaction. Using Ny6 and PBT samples bearing specific reactive end groups, it was demonstrated that only the carboxyl end groups of PBT and of Ny6 are able to react in the initially biphasic Ny6/PBT blends, so that an outer-inner exchange takes place (Scheme 3). The composition of the copoly(esteramide) obtained at 260degreesC in the exchange of Ny6/PBT shows a higher amount of Ny6 units with respect to the initial blend composition and a high value of the Ny6 average sequence length. However, the composition of the copoly(esteramide) obtained at 280degreesC was instead found to be equal to the feed composition (50/50), with a random distribution of average sequence lengths. The results obtained in the present work can be reconciled within the overall model of exchange reaction occurring through active chain ends.