Different cellulose substrates and organosolv lignin were treated heterogeneously with organic solutions of trialkoxysilanes bearing vari- able function on the forth substituent. It was shown unambiguously for the first time that Si–OR does not react with the hydroxy groups of cellulose even at high temperature, whereas it condenses with the phenolic OH of lignin. The addition of moisture to these systems induces the partial hydrolysis of the siloxane moieties and the ensuing silanol groups can then react with the cellulose OH, but only at high tempera- ture. Using the latter systems and a siloxane bearing a polymerizable function, it was possible to attach poly(methylmethacrylate) chains to the surface of cellulose fibres through a two-step procedure.