A study has been made of the influence of transesterification on the miscibility in binary blends of poly(ethylene terephthalate) and poly(bisphenol-A carbonate). The blends were melt mixed in the range 260–300°C and studied by differential scanning calorimetry, dynamic mechanic analysis, and by Fourier transform infrared spectrometry. It was found that copolymer produced by a transesterification reaction can enhance the miscibility of this system. The new compositions were uniquely identified by FTIR. Gel permeation chromatography showed that molecular weight decreases were not the origin of miscibility. The ester exchange reaction itself was found to be initiated by the residual catalyst in the commercial polymers selected for study. This has been shown by the absence of reaction after polymer purification by solution and reprecipitation prior to melt mixing.