A method of preparing a sheet of a lignocellulosic material such as paper for the manufacture of a finished product, includes the steps of impregnating the sheet with an impregnating composition comprising a composition for the chemical modification of the lignocellulosic material containing a dicarboxylic anhydride or a tricarboxylic anhydride dissolved in a suitable non-aqueous solvent, and a composition for resinating the lignocellulosic material containing an isocyanate thermosetting resin dissolved in a suitable non-aqueous solvent. Thereafter any excess of the impregnating composition is removed from the impregnated lignocellulosic material and the non-aqueous solvent or solvents are removed. Subsequently the sheet of lignocellulosic material so treated may be used in a method of forming an article by adhering a sheet so treated to a second sheet optionally so treated, with a suitable adhesive material. The result is a product which may be used as a building board or the like.