The reaction of a cellulosic material (Whatman no. 42 filter paper) with two silanes was studied by Diffuse Reflectance Ultraviolet-Visible Spectroscopy and Diffuse Reflectance Fourier Transform Infrared Spectroscopy. The silanes were 3-aminopropyltriethoxysilane (APS) and methyltrimethoxysilane (MS), a silane with a non-reactive organofunctional group. The use of organic peroxides was required in order to achieve an efficient cellulose modification with the silanes. The Diffuse Reflectance UV-VIS Spectroscopy can provide valuable information about the mechanism of the reactions between silanes and cellulose. The reaction between APS and cellulose involves not only the silanol groups of the silane, but also the amino groups. The spectra of APS-trealed celluloses show an absorption band at 325 nm. This band was assigned to imines produced in the reaction of the amino groups of APS with the cellulose. This absorption band increased when APS-treated cellulose was cured at 120°C. The amino groups attached to the cellulose dissappeared in this process