The effects of interchange reactions on the solid-state structure and mechanical properties of a 70/30 poly(ethylene terephthalate) (PET)/bisphenol A polycarbonate (PC) blend were studied. Increasing reaction levels were obtained by means of lower screw speeds in the extruder. The progressive production of copolymers with the reaction time increased the amount of each component in the other phase. The concomitant degradation of PET led to a maximum in ductility and tensile and impact strengths whereas the modulus of elasticity and the yield stress were held constant. The maximum in properties took. place at a reaction time close to 2.6 min; at longer reaction times the negative effect of degradation began to overcome the positive effect of the interchange reactions.