The present study investigated the dependence of various mechanical and fracture properties on the volume fraction, phi(f), of the reinforcing glass fibres in Polycarbonate/Acylonitrile Butadiene Styrene (ABS) blends. The addition of glass fibres enhanced the ultimate tensile strength and modulus and reduced elongation (both to yield and to break) and total work of fracture. The elastic modulus was not significantly affected by the loading mode although the ultimate strength was significantly affected, giving flexural strength values of 1.5-1.6 times greater than tensile strengths. The elastic modulus and strength were linear functions of phi(f) and thus followed the principle of rule of mixtures; The presence of weldlines in specimens had an adverse effect on most tensile properties except for the elastic modulus. Linear elastic fracture mechanics could not be used to assess the resistance to crack propagation of the present range of materials because their behaviour violated the principle assumptions upon which the theory is based. An alternative method was employed where the total work of fracture and the work of fracture corresponding to the maximum load were plotted as a function of initial crack length. These plots were reasonably linear for the polymer and its composites giving values of the resistance to steady state crack propagation J(T) and the J integral of maximum load J(m) respectively. Values of J(T) and J(m) decreased with increasing phi(f).