Polyethylene terephthalate (PET) was blended with two kinds of co[poly(ethylene terephthalate-p-oxybenzoate)] (POB-PET) copolyester, designated as P46 and P64, respectively. The PET and POB-PET copolyester were combined in the ratios of 85/15, 70/30, and 50/50. The blends were melt mixed in a Brabender Plasticorder at 275, 285, and 293 degrees C for different amounts of time. The transesterification reactions during the melt mixing processes of PET with POB-PET copolyester blends were detected by proton nuclear magnetic resonance analysis. The values of the rate constants are a function of temperature and the composition of blends. The transesterification reactions that may occur during the melt mixing processes have been discussed also.