Composites consisting of synthetic polymers and natural organic fillers have many applications. Plant-based materials, particularly wood, are used as fillers. Most frequently they are added to the polymer in the shape of wood flour or wood fibres. This type of polymer composite allows the possibility of modifying the components in accordance with the requirements of the application. Modifications to wood flour (WF) were carried out with a 10% solution of maleic anhydride (MA) in acetone in the presence of benzoyl peroxide and dicumene hydroperoxide. Lignin and cellulose were isolated from the WF and modified by the same method as the WF. They were characterized by Fourier Transform infrared spectroscopy and it was proved that there was a chemical reaction between the wood components and MA. This was confirmed by chemical analysis and by a determination of the hydroxyl groups in unmodified and modified WF. Composites of polypropylene and modified WF were obtained and studied by scanning electron microscopy, transmission electron microscopy, dynamic mechanical analysis and mechanical tests. They showed an improvement in adhesion between the polymer and the modified WF