In reactive extrusion, the extruder is used as a solvent-free continuous chemical reactor able to process highly viscous materials. The chemical transformation of biopolymers by reactive extrusion appears as a very promising technology. Although punctual applications in this field have already been achieved on a laboratory or pilot scale, the amount of work to carry out is still considerable. A wide range of reactions and raw materials may be explored, and the reactions achieved on a laboratory scale have to be optimized and transposed to an industrial scale. Process modelling and simulation constitute useful tools for process understanding, development, optimization and scale-up. Although reactive extrusion modelling has interested many authors, it still remains a challenge because of the complex geometry and the strong coupling between operating parameters, flow conditions, material rheological behavior and reaction kinetics. A steady-state mathematical model for a biopolymer oxidation process by reactive extrusion is here proposed. The model is based on a hybrid approach combining chemical engineering methods and simplified continuum mechanics laws. The combination of these two approaches enables to simplify the calculations related to chemical reactions while ensuring a predictive character. The flexible structure of the model enabled its implementation within a global process simulator. A method to minimize the amount of experimental data required for model parameter adjustment is also presented. The model was validated by experiments conducted on a semi-pilot corotating twin-screw extruder. Even if it may be refined, the model proposed already constitutes a useful tool for later research work dealing with the development, modelling and simulation of chemical reactions in corotating twin-screw extruders.