The performance of fiber-reinforced polymer matrix composites is strongly determined by the interfacial adhesion between fiber and matrix. As polymer matrix and inorganic glass fibers are chemically different, glass fiber sizing has to be employed to enhance fiber matrix adhesion. A new approach to optimize fiber matrix adhesion is an atmospheric pressure plasma treatment of the glass fiber reinforcement resulting in modifying and activating the glass fiber size. It is well known that adhesion can be improved by plasma treatment. To identify how the plasma acts and how the size is modified by the plasma, a commercial silane size was treated with a dielectric barrier discharge in air and analyzed by X-ray photoelectron, IR, NMR and Rutherford backscattering spectroscopy. The appearance of new OH, ester, carboxyl groups was observed. The modification consisted mainly of polyether chain scissions as a consequence of the formation of hydroperoxides. (C) Koninklijke Brill NV, Leiden, 2012