A substrate containing a natural polymeric material is modified by: A) treating the substrate containing the natural polymeric material with a modifying agent selected from the group consisting of organo-functional coupling agents and multi-functional amine containing organic compounds; and B) optionally exposing the substrate containing natural polymeric material with one or more treatments selected from the consisting of: i) subjecting the substrate to extraction with a solvent to reduce the content of extractable materials associated with the natural polymeric material prior to or during treatment with the modifying agent; ii) treatment with a physical field selected from static physical fields, high-frequency alternating physical fields and combinations of two or more thereof either prior to, during or after treatment with the modifying agent; and iii) oxidation of at least part of the natural polymeric material prior to or during treatment with the modifying agent.