The flexural properties of isotactic polypropylene (PP) matrix composites reinforced with 5Β±30 vol% of unidirectional pitch-based carbon, polyacrylonitrile (PAN)-based carbon, e-glass or aramid fibers were measured using both static and dynamic test methods. Previous research has shown that these pitch-based carbon and aramid fibers are capable of densely nucleating PP crystals at the fiber surface, leading to the growth of an oriented interphase termed a Βͺtranscrystalline layerΒΊ (TCL), while the e-glass and PAN-based carbon fibers show no nucleating ability. The PP matrices examined included unmodified homopolymers, nucleated homopolymers and PP grafted with maleic anhydride (MA). The composites based on the unmodified PP homopolymers all exhibited poor fiber/matrix adhesion, regardless of fiber type and presence or absence of a TCL. The addition of nucleating agent to the PP matrix had no measurable effect on either the amount of TCL material in pitch-based carbon-fiber-reinforced composites, as measured by wide-angle X-ray scattering, WAXS, or the static flexural properties of the composites reinforced with either type of carbon fiber. However, MA grafting reduced the transcrystalline fraction of the matrix in pitch-based carbon-fiber-reinforced composites; at the highest level of MA grafting, the TCL was completely suppressed. In addition, high levels of MA grafting improved the transverse flexural modulus of the composites containing both types of carbon fibers, and reduced the extent of fiber pull-out, indicating an improvement in fiber/matrix adhesion.