The modification of viscose cellulosic fibers from eucalyptus wood was performed by alkali treatment to improve the surface properties of the fibers for subsequent incorporation as reinforcement into phenolic composites. The treatment was carried out at various NaOH concentrations (1-20 wt %) and soaking times (1 and 2 h). The structural transformations of the fibers were determined by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Morphological observations of the fibers were performed using scanning electron microscopy (SEM), and wettability between the fibers and a resol-type phenolic resin was studied by contact angle measurements. Thermogravimetric analysis (TGA) was used to determine the thermal properties. The treatment of cellulosic fibers with 5 wt % NaOH for 2 h was selected as optimum. According to the analyses, these conditions increase the amorphous regions of the fibers (FTIR), reduce the crystallinity (XRD), swell the microfibers and fibers (SEM), and improve the wettability and the thermal stability. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2198-2204, 2013