The intermolecular interchange reaction between poly(ethylene terephthalate) and poly(butylene terephthalate) was investigated by a spectroscopic method. The rates of direct ester-ester interchange and alcoholysis reactions were determined by using newly derived rate equations and analytical data of heterolinkages formed. The characterization of heterolinkages formed during the interchange reaction was carried out by means of carbon-13 UWR spectroscopy. It is concluded that the direct ester-ester interchange reaction is more prevailing than the alcoholysis reaction.