Ion irradiation has been investigated as a route for the preparation of mechanically stable and highly durable cross linked sulphonated PEEK for fuel cell application. The formation of cross linking was confirmed by solvent extraction using the well known Charlsby–Pinner equation. The DSC results indicated that the cross links retard the crystallization, but no changes were observed in the mechanism of crystallization. The thermal kinetic data for irradiated PEEK quantitatively suggest that these films still have sufficient thermal stability for long term applications as fuel cell membranes. Analysis of the dielectric response by Cole-Cole, Havrilak and Negami and Williams and Watts equations indicated that the dipole relaxation was broadened and becoming more asymmetric with cross link density. The sulphonation of the cross linked PEEK in concentrated sulphuric acid indicated that the rate of reaction decreased with cross linking density and was consistent with diffusion control kinetics. Increasing cross link density resulted in more bound water in the equilibrated membranes and the nano-structure present was comparable to that of Nafion. Cross linking improved the chemical stability of PEMs in particular in methanol solution.The measurement of power output and energy efficiency suggested that the cross linked PEMs produced are promising candidates to replace Nafion membranes but more information are required, in particular on their long term stability under fuel cell operating conditions.