The adsorption of several prehydrolyzed alkoxysilanes onto the surface of cellulosic fibers in ethanol/water mixtures has been studied quantitatively. The corresponding isotherms show the formation of a monolayer followed by further adsorption. The nature of the fourth substituent on the silane plays a specific role on the features of this association. Whereas at room temperature only dispersive and polar attractive interactions occur, heating above 100 degreesC produces condensation reactions resulting in covalent bonding between the substrate and the silane forming the monolayer. As a result of this investigation, the possibility of calling upon this surface modification in order to compatibilize cellulosic fibers with polyolefinic matrixes for the elaboration of composite materials seems to be a promising strategy.