The use of two polycarboxylic acids (PCAs), such as 1,2,3,4-butanetetracarboxylic acid (BTCA) and citric acid (CA), was employed to esterify handsheets prepared from thermomechanical pulp (TMP) fibers. Sodium hypophosphite was used as a catalyst. The Fourier transform infrared (FTIR) spectrometry technique was utilized to verify the ability of these PCAs to form ester functions between the fibers. The modifications of both the temperature and the mass amount of the PCA were investigated. Finally, the wet breaking length of the prefabricated and cured handsheets was measured and an improvement was observed in both cases. The raise of the mass amount increased the wet breaking length for the two cases. On the other hand, a comparison between the two PCAs was carried out. For the same mass amount, BTCA was more effective to cross-link the TMP fibers at high temperature while CA showed slightly better results at low temperature.