The effects of various thermal treatments on the properties of both PEEK and APC2 have been systematically studied. X-ray diffraction data, together with dynamic mechanical measurements, indicate that the T(g) values for both materials are considerably higher than those generally reported. Also, the tan delta values for quenched APC2 specimens are considerably higher than for slowly cooled materials. Microstructural observations show that this is likely to be due to poor fibre/matrix adhesion, in the case of the quenched specimens. The link between tan delta and degree of fibre/matrix coupling is currently being explored.