AbstractSilane coupling agents are but one of many ingredients in commercial sizings that are applied to glass fibers. The chemistry of a silane coupling agent alone allows it to react with both the glass fiber surface and the epoxy matrix to increase the fiber/matrix adhesion. However, the action of a commercial sizing system containing a silane coupling agent along with other components is not well understood. Research has been conducted in which the physical properties have been measured of blends of epoxy‐compatible silane/sizing made with bulk matrix at concentrations representing likely compositions found at the fiber‐matrix interphase as a result of processing and fabrication. It has been shown that the silane/sizing interaction with the epoxy matrix produces a material with vastly different properties than those of the bulk matrix. In this particular system, the model interphase has a lower Tg higher modulus, and greater tensile strength, but lower toughness. The results from the present study show that a themical interaction theory of adhesion alone is not sufficient to explain the role of silane coupling agents in glass fiber‐epoxy matrix adhesion. Consideration must be given to the interphase and its mechanical properties.