The rate equation of interchange reaction between poly(ethylene terephthalate) (PET) and poly(bury-lone terephthalate) (PBT) was derided considering the thermal degradation of heterolinkages formed. Experimental results fitted well with the revised equation. The change in molecular weight of PET and PBT during the melt processing was examined. It was found that the molecular weight of PBT decreased with the melt processing time. but that of PET increased with the time.