The esterification reaction between the hydroxyl end groups in PET and trifluoroacetic acid on stirring at room temperature has been observed and studied. The reaction, which we refer to as end-capping, was found to be complete after 5 days. We demonstrate that whilst the reaction leads to modification of the end groups there is no significant degradation of the polymer chains. Proton nuclear magnetic resonance has been used to probe the kinetics of the reaction, and a pseudo first-order reaction rate constant of (7.8 +/- 1.8) x 10(-6) s(-1) has been obtained. The results of this study may have implications for other polyester systems where strong acids are used as solvents.