The effect of irradiating amorphous poly (ether ether ketone), PEEK, with ions, 11 MeV proton (H+), and 25.6 MeV helium (He2+), has been investigated focusing on the changes in thermal properties. The extent of chain scission and crosslinking was evaluated using the Charlesby-Pinner equation. Crosslinking increased the glass transition temperature (T-g) in line with the DiBenedetto equation from which the crosslinking constant for each ion was calculated. The effect of irradiation on the thermal degradation kinetics was studied in an argon atmosphere at a constant heating rate by mean of the Chang and the second Kissinger methods. Irradiation significantly reduced the thermal stability of the polymer and its service lifetime. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2212-2221, 2008