The present study investigated the dependence of various mechanical and fracture properties on the volume fraction, phi(f), of reinforcing glass fibres in acrylonitrile-styrene-acrylate (ASA) copolymer. The addition of glass fibres enhanced the ultimate strength and modulus as measured in both tension and flexure but reduced the total work of fracture. The elastic modulus was not affected by the loading mode. The ultimate strength in flexure was found to be always greater than in tension by a factor of about 1.3. Both properties were found to be a linear function of phi(f) following the rule of mixtures: P-c = lambda P(f)phi(f) + P-m (1 - phi(f)) where P-c is the measured property for the composite, P-f and P-m are the corresponding values for the fibre and the matrix, respectively, and lambda is the overall efficiency of the reinforcing fibres. Addition of glass fibres to ASA polymer reduced both the notched and the unnotched impact strengths. Linear elastic fracture mechanics were used to determine values of the fracture toughness and the strain energy release rate. The fracture toughness did not change significantly with phi(f), whereas the strain energy release rate decreased with increasing phi(f). The presence of weldlines in the specimens had an adverse effect on all tensile properties except for the elastic modulus. The weldline integrity parameter for the modulus was between 1 and 0.95, and for strength it was between 0.87 a nd 0.20, decreasing ii nearly with increasing phi(f).