Improving thermal stability of TEMPO-oxidized cellulose nanofibrils (TOCNs) is a major challenge for the development and preparation of new nanocomposites. However, thermal degradation of TOCNs occurs at 220 degrees C. The present study reports a simple way to improve thermal stability of TOCNs by the heat-induced conversion of ionic bonds to amide bonds. Coupling amine-terminated polyethylene glycol to the TOCNs is performed through ionic bond formation. Films are produced from the dispersions by the casting method. Infrared spectroscopy and thermogravimetric analysis confirm conversion of ionic bonds to amide bonds for the modified TOCN samples after heating. As a result, improvement of TOCNs' thermal stability by up to 90 degrees C is successfully achieved.