The mechanical properties of polyetheretherketone and polyetherketone were studied asa function of strain rate and temperature in the ranges 10(-3) to 10(3) s(-1) and 20-200 degrees C. At temperatures below the glass transition temperature, T-g, the strain-rate sensitivity of both polymers was found to be almost independent of temperature with a value of similar to 4MPa per decade of strain rate and the behaviour was well described by an Eyring relationship leading to an activation volume of similar to 1 nm(3). Above T-g, X-ray and differential scanning calorimetry studies show that cold-crystallization phenomena play a very important part in the polymer behaviour, leading to an increase of yield stress with increasing temperature, The crystallization was found to be highly strain-rate dependent with no increase in crystalline content occurring in quasi-static tests and increases of up to 20% in higher rate tests. Much of the data have been gathered using novel test equipment which is described in the paper.