Method of treating cellulose fibers wherein the fibers are treated with a composition containing a reagent A-B-X based on cyanuric chloride (A) in which one of the three chlorines has been reacted with an amine or hydroxyl compound (B-X). The reagent reacts under alkaline conditions with cellulose hydroxyl groups. A resin, e.g., polyester or vinyl resin, is then reacted with the thus modified cellulose fibers through the B-X substituent.