to increase interfacial strength, the research described focuses on modifying the fiber surface (via high-temperature acid treatment or zinc electrolysis) to facilitate chemical functional groups on the fiber that might increase fiber-matrix interactions the thermoplastic matrix materials used were random copolymers of ethylene and methacrylic acid in which the carboxyl groups in the methacrylic acid segments were neutralized with either sodium or zinc counter-ions. mechanical tests were performed to determine the macroscopic effects of fiber pretreatment on the ultimate mechanical properties of the composites. fabrication was designed such that fiber-matrix separation provides the dominant contribution to mechanical fracture. the presence of acid functionality in the matrix, rather than the divalent nature of the zinc counterions, produces the largest relative enhancement of transverse (tensile) fracture stress in the composites containing surface-treated carbon fibers