The transesterification reaction of poly(ethylene terephthalate)/poly(ethylene 2,6-naphthalate) blends during melt-mixing was studied as a function of blending temperature, blending time, blend composition, processing equipment, and different grades of poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalate), Results show that the major factors controlling the reaction are the temperature and time of blending. Efficiency of mixing also plays an important role in transesterification. The reaction kinetics can be modeled using a second-order direct ester-ester interchange reaction. The rate constant (h) was found to have a minimum value at an intermediate PEN content and the activation energy of the rate constant was calculated to be 140 kJ/mol.