Sisal fibers were benzylated under different conditions and were characterized with infrared spectroscopy, X-ray diffraction, thermal analysis, and scanning electron microscopy. The benzylation reaction was monitored by the mass gain as a function of the reaction time. In the first stage, there was a mass loss associated with the loss of lignin and polyoses from the raw fiber, which was accompanied by an increase in mass due to benzyl incorporation. When fiber delignification was carried out before benzylation, the mass gain curves of the benzylation reaction presented no initial mass loss and a much higher mass gain. Benzylation promoted several morphological changes: (1) the loss of the parenchyma cells, (2) the defibrillation of the technical fibers into ultimate fibers, (3) the microdefibrillation of the ultimate fibers, and (4) benzyl incorporation. The crystallinity of the fibers decreased with benzylation, as observed by X-ray diffraction. The thermal stability of the fibers varied according to the treatment used. Other changes promoted in the fibers by chemical modification were examined. (C) 2003 Wiley Periodicals, Inc.