Blends were made of polycarbonate (PC) and polybutylene terephthalate (PET) or amide modified polybutylene terephthalate (PBTA). The blends were prepared by solution casting, solution precipitation and melt mixing. The transesterification interchange reactions were followed by H-1 n.m.r. analysis. The change in the glass transition temperature (T-g) of PC has been studied as function of blending conditions, the addition of titanate catalyst inactivator, the content of Li-residues and the molecular weight of the polymers. A correlation is made between the degree of transesterification and the lowering of the glass transition. Triphenyl phosphite (TPPi) (0-2.2 wt%) was used as titanate catalyst inactivator. TPPi was found to be an effective catalyst inactivator however, this low molecular weight compound has also a strong plasticizing effect on PC. A concentration of 0.5 wt% TPPi gave a good stabilization against transesterification. Without transesterification the PBTA was found to be less miscible with PC than PET and the PC/PBTA blends had higher TgS for the PC phase than PC/PBT and a higher melting temperature of the system.