A study was made of the strength and relaxation mechanical properties of block samples of polyketones and of polyketone composite materials. In addition, the mechanical properties of block samples of blends of polyketones and polysulphones were investigated. The samples were prepared by press moulding and injection moulding. The structure of the polyketones under study is partially crystalline, resulting in the appearance of two regions of mechanical serviceability. On the one hand there is the low-temperature region, in which the material withstands high mechanical stress levels without softening or failing; on the other hand there is the high-temperature region, where the material withstands relatively low levels of stress, but remains serviceable at elevated temperatures. The introduction of short-chopped glass fibre results in higher strength, and reduces the stress relaxation intensity; the introduction of polysulphone suppresses crystallization of polyketone by changing the mode of its relaxation behaviour. It is shown that polyketone based on iso-phthalic and tere-phthalic acids and on the polyketone-polysulphone blends, when processed by injection moulding, result in samples that have good mechanical properties over a rather wide range of temperature.