Mechanisms operating in the exchange reactions occurring in the melt mixing processes of Bisphenol A polycarbonate (PC) with poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) blends have been investigated making use of appropriate polymer samples, capped or containing reactive chain end groups. The exchange process may proceed by two different mechanisms : a direct exchange reaction between inner functional groups located inside the polymer chains, i.e., inner-inner, or by attack of reactive chain ends functional groups (outer) on inner groups, i.e., outer-inner. It is shown that the distinction between the two processes can be conveniently made by determining the composition of the copolymer formed in the exchange reaction. The inner-inner mechanism occurs only in the reaction between end-capped or high molar mass PET/PC or PET/PC samples, and it was found that the molar composition of the copolymer formed is always equal to the feed ratio of the two homopolymers and independent from the reaction time. The outer-inner mechanism occurs in the presence of hydroxyl or carboxyl reactive chain ends in PBT and PET samples. The reaction proceeds by the attack of the reactive end groups on the PC chains, originating block copolymers of PC and PBT and low molar mass PC with phenol end groups which are unreactive. The reaction stops right after the reactive end groups are consumed. The amount and the composition of the copolymers generated in the reactions are found to be constant as a function of time. The copolymer composition shows an excess of PBT or PET units with respect to the feed molar ratio. These results indicate that monitoring the composition of the copolymer formed in each case is diagnostic for establishing the mechanism of the reaction. The approach used here allows control of the composition and yield of the copolymer to be produced, and it is applicable to other systems where exchange reactions occur.