The kinetics of the trans esterification reaction between poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) with and without the addition of a chain extender were studied with H-1 NMR. Different kinetic approaches were considered, and a second-order, reversible reaction was accepted for the PET/PEN reactive blend system. The addition of 2,2'-bis(1,3-oxazoline) (BOZ) promoted the transesterification reaction between PET and PEN in the molten state. The activation energy of the transesterification reaction for the PET/PEN reactive blend with BOZ (94.0 kJ/mol) was lower than that without BOZ (168.9KJ/mol). The rate constant k took an almost constant value for blend samples with different compositions mixed at 275 degreesC.