The extent of transesterification in poly(ethylene terephthalate) (PET)/poly(ethylene-2,6-naphthalate) (PEN) blends with the addition of PET-PEN copolymers was examined by DSC and H-1-NMR measurements to evaluate the factor affecting the reaction level at a given temperature and time. Both block (P(ET-block-EN)) and random (P(ET-ran-EN)) copolymers were used as the copolymers. At a given treatment temperature and time, the level was increased by the addition of P(ET-block-EN) into PET/PEN blends. On the other hand, a reverse change was observed when P(ET-ran-EN) was mixed with PET/PEN blends. During the treatment, an inhomogeneous phase of the blends changed into the homogeneous one; however, the change showed little effect on the reaction level. The effects of molecular weight on the reaction level were also examined. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 2716-2723, 2009