The interfacial properties can also be studied by means of post-mortem fractography. Fractographic analysis shows convincing indications of a stronger interfacial bond for the glass-fibre/MA-PP material, since the fibres were covered with residual polymer matrix. The stronger interfacial bond is due to the affinity of the maleic anhydride groups to the fibre sizing. The glass-fibre/PP did not show any polymer residue on the fibres. The interfacial bond is here probably formed by interdiffusion of molecular segments into the sizing, with weak van der Waals bonds on the molecular level. Compressive stresses provide additional positive effects.