Extrusion is a versatile process that has developed beyond its original uses for metals and plastics. Today, twin-screw extrusion is also used in non-plastic applications, e.g. processing of food, animal feed or refining of biomaterials. Reactive extrusion is regarded as a green route to chemical modifications, as it enables solvent-free reactions. The aim of our work was to produce functionalised bio materials that enable more tailored or targeted preparation of new bio-composites or their use for subsequent derivatisations. We used twin-screw extrusion to functionalise wood particles (WP), cellulose and hemicellulose. These materials were processed without any polymeric carrier, unlike other published work in this area. Succinic, dodecenyl succinic and maleic anhydride were used as reactants and under optimum conditions, esters between these anhydride and hydroxyl groups of the biomaterials were formed. The influence of extrusion conditions and catalyst on the reaction yield was assessed by infrared and nuclear magnetic resonance spectroscopy, thermal analysis and acid titration. Based on the acid titration, the highest degree of esterification was obtained for the succinilation of cellulose (1.4 mmol/g dry wood). Mass balance results suggest an almost complete conversion of succinic anhydride, indicating the formation of di-esters. The order of reactivity for succinic anhydrides was cellulose > WP > hemicellulose, while for maleic anhydride the order was WP > hemicellulose > cellulose. In the absence of catalyst, higher temperatures increased the yield from 1.1 to 1.4 mmol/g wood between 150 and 220°C, respectively. The yield was independent of the extrusion temperatures assessed in formulations with catalyst. The shear energy during extrusion led to a reduction in particle size, defibrillation and compaction as confirmed of the fibres by microscopy.