High‐modulus carbon‐fiber‐reinforced thermoplastic composites typically fail at the interface due to poor adhesion between fiber and matrix. To increase interfacial strength, the research described herein 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 inter‐actions. The thermoplastic matrix materials used in this study 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 counterions. 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 gracture. Composites containing fibers treated with nitric acid, or a mixture of nitric and sulfuric acids exhibit a 20 to 25 percent increase in transverse (tensile) fracture stress relative to composites fabricated with as‐received fibers. Scanning electron microscopy of the fiber‐matrix interface at fracture allows one to “zoom‐in” and obtain qualitative details related to adhesion. Fracture surface micrographs of the above‐mentioned acid‐treated fiber‐reinforced composites reveal an increase in the amount of matrix material that adhered to the fiber surface relative to the appearance of the fracture surface of composites fabricated with as‐received fibers. 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 above‐mentioned composites containing surface‐treated carbon fibers.