A reactive sizing was designed to achieve high levels of interfacial adhesion and improved mechanical properties with a carbon fiber acrylate system cured by electron beam (EB). The sizing was made of a partially cured epoxy sizing with a high density of pendant functional groups (acrylate functionality) to facilitate covalent bonding with the matrix. The interlaminar shear strength improved from 61 to 81 MPa (+33%) without postprocessing, reaching a shear strength similar to that of the same system cured by a thermal treatment. Observation of the fracture profiles clearly highlighted a change in the fracture mechanism from a purely adhesive failure to a cohesive failure. To the best of our knowledge, such improvements of the mechanical properties of carbon fiber composites cured by EB, without any postcure, have not been reported previously. This constitutes a breakthrough for the industrial development of EB curing of composites.