The effect of crosslinks introduced by ion irradiation with 11.7 MeV proton and 30 MeV helium ions on the reactivity of poly(ether-ether-ketone) (PEEK) to sulfonation have been investigated following the kinetics of the reaction at room temperature. Concentrated sulfuric acid was used as a swelling and sulfonating agent and the reaction was followed by changes in the FTIR spectrum. The rate of reaction decreased with the degree of crosslinking and the progress with time was consistent with diffusion control of the sulfuric acid into the crosslinked matrix. The results were consistent with the efficiency of the ions in crosslinking PEEK and in particular with the differences in their linear energy transfer (LET). (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 775-783, 2009