This article reports on the results of a collaborative work aiming at improving the mechanical performances of composites based on flax fabrics and epoxy thermosets or silicone materials. Radiation processing was assessed as a simple pretreatment to modify the surface composition of commercially available fabrics. Two approaches were explored after analysis of the chemical features of the two types of composites. A first method designed for epoxy thermosets is based on the immobilization of bio-based adhesion promoters, partially epoxidized fatty oils exhibiting carbon-carbon unsaturations and epoxy groups expected to act as coupling agents after simultaneous EB radiation-grafting process. The second method was based on the peroxidation by EB-irradiation in air of the flax fabrics selected for the reinforcement of vinylsiloxane-based silicone materials. The thermal decomposition of peroxyl groups during the curing of a silicone matrix including vinylsiloxane units was indeed expected to generate covalent links between the fibers and reactive silicone formulations. The advantage and limitations of the two methods are discussed based on mechanical testing of composite specimens fabricated with the original fabrics, or after implementation of the radiation-induced pre-treatments. The efficiency of the proposed grafting processes is supported by XPS at the different stages of the modification process, as well as by SEM observation of fractured samples. The spectroscopic signature of adhesion promoters for the epoxy composites or of models of the silicone matrix was observed after radiation-promoted immobilization and extensive washing of fabric samples.