Acetylation is a common way to enhance interfacial adhesion between natural fibres and polymeric matrices in composites. This work investigates the effect of a solvent-free environmentally friendly acetylation treatment of short flax and kraft fibres on the impregnation behaviour and tensile properties of flax fibre/epoxy composites. Untreated and acetylated fibres have been used to prepare fibre mats, which were next studied for their morphology, their permeability to liquid epoxy, and their ability to adhere to the epoxy matrix after cure. The results show that acetylation enhances the roughness of flax fibres and improves fibre-matrix interaction, but reduces the permeability of the fibre mats and the intrinsic properties of fibres, thus leading to slightly lower composite modulus and strength. The proposed treatment is an interesting option to improve the affinity between natural fibres and polymeric matrices, but more work will be required to improve the interfacial adhesion without damaging the fibres.