The effects of blending tim on the ester interchange reactions between poly-ethylene terephthalate) (PET) and polycaprolactone (PCL) during melt blending were studied. The block lengths of ET and CL units of copolymers formed via interchange reactions were characterized by H-NMR analysis of the bulk product and fractionaed samples. The P(ET/CL) copolymers were also characterized by DSC, X-ray, SEM analysis and biodegradation tests. New peaks in the H-NMR spectra of copolymers, assigned to the methylene hydrogen adjacent to the ester group whose environment is different from those in PET or PCL, suggest that P(ET/CL) copolymers are formed through ester interchange reaction during melt blending. However, ester interchange reaction does not occur significantly at short melt blending times of less than 15 minutes. The melting temperatures and heats of fusion assigned to ET-like unites in P(ET/CL) copolyesters decrease with blending time, and the melting peak of CL-like units can not be distinguished in samples melt blended over 20 minutes, in which case the average CL and ET segment lengths are approximately 5. The TOC of the copolymers increases sharply with increase in melt blending time below 30 minutes and gradually thereafter. Increases in the accessibility of the CL chains with decrease in crystallinity appears to be the significant factor in the biodegradation behavior of the copolymers.