AbstractMelt blends of polycarbonate and poly(ethylene terephthalate) were prepared and examined for their transitional behavior by thermal analysis and dynamic mechanical testing. A singleTgwas observed for compositions containing more than 60%–70% PET by weight while compositions below this range showed two glass transitions. From this it is concluded that PC and PET are completely miscible in the amorphous phase for PET‐rich compositions, whereas PC‐rich blends separate into two amorphous phases which apparently contain both components. Melting point and crystallization behavior are conssistent with these conclusions and suggest that very little if any interchange reactions occur between the ester and carbonate groups during melt m