Sulphonated PEEK polymers with a different sulphonation degree (DS) were obtained by varying the sulphonation reaction temperature. Ionomeric membranes were prepared by using different solvents (DMSO, DMAc, DMF and DMAc/DMF mixture). Structural and chemical-physical film characterisations were carried out by X-ray and thermo gravimetric analyses. Membranes obtained from DMF and/or DMAc have a low crystallinity with an amorphous structure, while the DMSO-sPEEK membrane has a more crystalline structure that could inhibit proton conduction. The efficient proton conducting structure for sPEEK chains seems to be the statistical coil. The analysed membranes do not present any ionomer or matrix peak associated with the clustering of sulphonic groups or phase separation due to the statistical distribution of the sulphonic groups on the main PEEK chain. The thermal behaviour was investigated by a differential scanning calorimetry (DSC). From a cooling analysis all the water contained in the membranes was shown to be strongly linked to sulphonic groups and of a non-freezable type. The influence of the casting solvent on the cell performance was studied. The developed membranes were also tested in H-2/air single cell in the temperature range 80-130 degrees C. A maximum power density of about 400 mW cm(-2) at 0.5 V was reached at 130 degrees C. (c) 2006 Published by Elsevier B.V.