The extent of transesterification reaction, intrinsic viscosity, weight-average molecular weight and hydrodynamic radius of poly(ethylene 2,6-naphthalate) (PEN)/poly(ethylene terephthalate) (PET) (50/50, w/w) blend were measured as a function of reaction time at 280 degrees C. In PEN/PET blend, the reactions between end groups as well as the transesterification should be considered since the molecular weight of the blend increases with reaction time.