Reactive poly(ethylene terephthalate) / polycarbonate (PET/PC) melting blends were prepared with different cobalt catalyst and polymer compositions. The presence of catalyst increased the miscibility of the 20/80 (wt%) blend, according to differential scanning calorimetry (DSC) and dynamic-mechanical analyses (DMA). The catalyst also promoted the amorphisation of the PET chains. Besides, the catalyst caused drastic lowering of both moduli (storage and loss) and melt flow rate (MFR) of some blends. The best mechanical and processing properties were achieved for blends without catalyst, indicating that transesterification reactions prejudice mechanical properties.