Much research related to the use of natural fibers in polymeric matrix composites has been developed. The presence of -OH groups in the chemical components of the natural fibers generates an important hydrophylic tendency that produces adhesion lacks with hydrophobic polymeric matrices. In this work natural fiber bundles mechanically extracted from both stem and bunch of cultivation banana wastes have been modified by both alkalization and silanization treatments. To evaluate the changes introduced by treatments on the chemical structure of fibers, Fourier-transform infrared spectrophotometry has been employed. The evaluation of advancing dynamic contact angles along with the determination of total surface free energy by using the Owens-Wendt method indicate that the treatments allow reduction of their hydrophilic tendency by alterations on the physicochemical characteristics of the fibers. This behavior is confirmed by the reduction of moisture uptake, analyzed by thermogravimetric analysis. Small differences on noncellulosic components of stem and bunch fiber bundles have been found. (C) 2004 Wiley Periodicals, Inc.