The effect of the interchange reactions in PBT/PAr blends on thermal and mechanical properties has been studied as well as the influence of the concentration of tetrabutyltitanate on these properties. These studies have been carried out by means of differential scanning calorimetry (DSC) and tensile tests. The results based on DSC studies allow us to conclude that the capacity of the transesterified blends to crystallize decreases when compared with the physical blends, due to the formation of copolymers. Furthermore, an enhanced effect is observed when the amount of the catalyst is increased. In addition, a slight decrease in the low deformation mechanical properties and a significant increase in the deformation at break is observed as a consequence of the interchange reactions. The presence of tetrabutyltitanate, which accelerates the interchange reactions, has a 2-fold effect on these properties. On the one hand, it enhances the aforementioned process, but on the other hand, the associated effect of the degradation reactions decreases the mechanical properties, particularly those of the deformation at break.