The dependence of the various mechanical and fracture properties on the volume fraction of short glass fibres in the styrene maleic anhydride (SMA) polymer was investigated. Special attention has been given to describing the dependence of various mechanical properties on the volume fraction of the glass fibres, phi(f) by way of the ru le of mixtures. It was found that, strength, elastic modulus and fracture toughness, all follow a simple rule-of-mixtures of the form Q(c) = lambda Q(f)phi(f) + Q(m)(1 - phi(f)), where Q(c) is the measured quantity for the composite, Q(m) and Q(f) are the corresponding values for the matrix and the fibre, respectively, and lambda is the overall efficiency of the fibres, taking into account the orientation and the length of the fibres in the composite. It was a Iso found that, while the presence of the weldline had no significant effect upon elastic modulus, its presence significantly reduced tensile strength and the fracture toughness of SMA and its composites.