A cellulosic material is treated by providing a cellulosic material such as wood or a wood composite, which is impregnated with an aqueous dispersion comprising a polymer, the polymer comprising an olefin-carboxylic acid copolymer. The cellulosic material is then impregnated with a modifying agent, the modifying agent comprising a polyamine having greater than or equal to 2 amine groups. The modifying agent at least partially crosslinks the polymer within the cellulosic material.