A method is provided of altering the chemical and/or physical functionality of a fibrous substrate (e.g. fibers, fabrics and fiber-resin composites) by the action of a polymeric composition containing at least one phosphono- or phosphino-component. The polymeric composition may include VPA, VDPA or hypophosphorous acid. The fibres may be cellulosic and/or non-cellulosic, or metal or ceramic. The method can be used, for example, to enhance the receptivity of a fibrous substrate to dyes.