Samples of injection moulded poly(aryl-ether-ether-ketone) (PEEK) resin with molecular weights covering the range, M(W) =15 000-50 000, have been subjected to a variety of thermal treatments, and the resulting material properties measured. It was found that crystallinity could be increased by annealing the mouldings, but also that an effective ceiling value depended on the molecular weight, i.e. the higher the M(W), the lower the maximum crystallinity. Tensile modulus and yield strength of the moulded polymer increased with increasing crystallinity, but the molecular weight had no detectable effect in these cases. In contrast, both crystallinity and molecular weight independently influenced the toughness. Toughness was found to increase with increasing molecular weight, and to decrease with increasing crystallinity. Implications of these effects on the injection moulding of components of PEEK are discussed.