A method of modifying a polymer having hydroxyl groups, selected from the group of polysaccharides and lignin, to give a modified polymer comprising the step of contacting said polymer with at least one organic phosphonate salt in order to chemically modify the polymer, said organic phosphonate salt being in a liquid phase. The method of polymer modification provides novel polymers. Modified polymers obtained from a polymer having been treated with at least one organic phosphonate salt are also disclosed. The modified polymers can be used as such or separated and optionally recovered from the solution, optionally being formed into particular materials or shapes.