At strain rates greater than about 10 the deformation of polymers is an adiabatic process which can lead to the generation of very high localized temperatures. These may be well in excess of the temperatures required for thermal decem position of the polymer to occur. It is speculated that gaseous products arising from such decomposition may act as the source of microcavities and craze initiators. Experiments are described in which the thermal decem position reaction parameters of polyetherketone, polyetheretherketone, and polysulphone are measured using differential scanning calorimetry and thermogravimetric analysis, and the decomposition products analysed by mass spectrometry. The results indicate that the proposed mechanism could act as a source of micropores with total pore volumes amounting to several per cent of the material volume.