The role of vinylchlorosilane coupling agents in creating an interphase between a polyester matrix and a reinforcing E-glass fiber was investigated. Measurements of the cure kinetics of the polyester resin, using differential scanning calorimetry, revealed that the presence of untreated E-glass surfaces retarded the cure reaction of the polyester, while treatment of the glass with reactive silanes enhanced the cure relative to the unfilled resin. Internal reflection infrared spectroscopy was used to study the permeation of polyester into a polysiloxane coating and the chemical reactions of the vinylsilanes and polyesters. It was found that a vinyltrichlorosilane coupling agent forms a relatively impermeable siloxane film on the fiber surface that probably reacts with the polyester at the siloxane/polyester interface. Octenyltrichlorosilane forms a siloxane layer that is permeated by the polyester and coreacts with it. The resulting interphase is extremely weak and debonds readily from the fiber. Methacryloxypropyltrichlorosilane forms a siloxane layer that is easily permeated by the polyester and reacts with it to form a mechanically-strong interphase. It was also found that the silane surface treatments reduced the stress transmission to the glass fibers, as determined from fiber fragmentation tests, and that the optically observed modes of failure were consistent with the observations of the internal reflection infrared and fiber fragmentation experiments. A finite element analysis of a single fiber embedded in a polymer matrix was used to simulate the effects of interphase toughness and stiffness on the mode of crack propagation from a broken fiber end While chemical bonding of the interphase to the fiber surface is a necessary condition for a strong, stable interface, it was found that the stress transmission to the fibers (i.e., the fiber efficiency) and the modes of crack propagation are controlled by the stiffness, fracture toughness and the thickness of the applied coatings.