The surface modification of cellulosic substrates in the form of powders and sheets was carried out using nonpolar oligomeric and polymeric reagents bearing isocyanate and anhydride reactive moieties. The characterization of these surfaces by inverse gas chromatography and by dynamic and equilibrium contact angle measurements, before and after the treatments, revealed that the lower the polarity of the coupling agent, the higher the loss of hydrophilic character of the substrate. In particular, the use of styrenic copolymers of rather high molecular weight gave rise to essentially dispersive-modified surfaces which should be highly suited for the preparation of composites based on nonpolar macromolecular matrices.