Melt blends of polycarbonate (PC) and poly(butylene terephthalate) (PBT) were characterized by their transesterification and crystallization behaviour using Fourier transform infrared spectroscopy (FTi.r.) as well as nuclear magnetic resonance (n.m.r.) spectroscopy and differential scanning calorimetry (d.s.c.). The transesterification can be analysed by FTi.r. spectroscopy using two different spectral features. The appearance of new bands is used for the direct proof, and changes of i.r. bands correlated with structural order for the indirect proof. The results from the PC-rich blend show that the indirect proof is very sensitive, but a detailed knowledge of the crystallinity is necessary for a correct analysis. The improvement of the compatibility of both components due to the formed copolymers has been observed in the blend PC/PBT 50/50 by i.r. microscopy. Furthermore, a splitting of the PBT carbonyl band was found and discussed. The changes of the observed two peaks correlate directly with the changes in the conformation of the tetramethylene sequences of PBT.