This paper reports a comparative study of the effects induced on sisal pulp fibers by three different laccase-based systems, namely, laccase sinapyl aldehyde, laccase ferulic acid, and laccase TEMPO systems, applied to perform biobleaching, biografting, and cellulose oxidation, respectively. (The abbreviations SLD, FRC, and TEMPO are used to represent sinapyl aldehyde, ferulic acid, and the 2,2,6,6-tetramethylpiperidine-1-oxyl free radical.) A novel aspect of this study was the use of thermogravimetric analysis (TGA) to monitor surface changes in cellulosic microfibrils during the enzyme treatments and gain a greater understanding of the mechanisms of action of the laccase-based systems. The different modes of action of the studied laccase-based systems reflected in the different degradation profiles of pulps after treatment. TGA showed laccase to modify the thermal degradation path of the initial pulp, increasing the proportion of cellulose degrading allow temperature. The addition of SLD resulted in virtually no change of the thermal degradation path of the initial pulp, indicating that the laccase SLD system basically exerted its action on the lignin component of fibers. In contrast to SLD, FRC was found to significantly increase the amount of the paracrystalline fraction of cellulose, probably as a consequence of its incorporation into fibers. The presence of TEMPO, especially under those conditions boosting the oxidative functionalization, was found to cause an intense degradation of cellulose and the formation of a substantial amount of amorphous cellulose degrading at low temperature. A novel aspect of the laccase TEMPO system was identified in this work: its ability to reduce the hexenuronic acids (HexA) content content of pulp, under specific reaction conditions.