Natural fibres have a high potential as reinforcement of polymer matrices, combining high specific strength and modulus with sustainable production and reasonable prices. When used in composites, the fibre-matrix interphase controls the overall performance. Interphases are smaller in thickness compared to the size of the reinforcing fibres. Nevertheless, a change in their structure and chemistry leads to significant changes in their overall performance. The focus of this study is a sustainable enzymatic treatment of flax fibres to replace non-ecological coupling agents. Fibres are treated with laccase from a Cerrena unicolour polypore fungus, which induces side-chain oxidation, demethylation and hydroxylation in lignin. [1] The induced functional groups may be used to covalently bind reactive species such as dopamine to the fibre which then improves the fibre matrix interaction. The modified fibre surface was analyzed by SEM and AFM. Single Fibre Pullout Tests and Single Fibre Fragmentation Tests were performed to determine the influence of laccase on the interfacial shear strength in epoxy. The results show no negative effect on fibre strength or modulus. However, treatment with laccase and dopamine affects the fibre morphology indicated by additional layers on the fibre surface in AFM and REM imaging. Pullout tests show a 36 % increase of interfacial shear strength of laccase-dopamine modified flax fibres compared to the non-functionalized reference fibre. For fragmentation tests, modified fibres show an increase in the amount of fibre breakage and a decrease in fibre pullout, both indicating higher fibre-matrix adhesion. Consequently, it is possible to increase the interfacial shear strength between flax fibre and epoxy matrix by laccase modification in the presence of dopamine. This offers a sustainable method for chemical modification of natural fibres without using petrol-based coupling agents. Further tests will include surface energy measurements and different treatment procedures. 1 Crestini C, Argyropoulos DS. The early oxidative biodegradation steps of residual kraft lignin models with laccase. Bioorg. Med. Chem., 1998, 6, p. 2161-2169.