Composite SPEEK based PEMs, containing tungstophosphoric acid (TPA)and modified with benzimidazole derivatives as cross-linking spacers, were prepared and characterized by a number of physico-chemical methods. It was observed, that the cross-linking agents strongly affect membrane properties, reducing their swelling, but at the same time decreasing their proton conductivity. However the cross-linkers provided a better retention of the TPA particles embedded within the composite membranes. The TPA particles enhanced the polymer conductivity to values overpassing that of the non-modified membranes. The performance of the PEMs was tested in hydrogen/air FC and found to be superior to that of Nafion 212 commercial PEM. The stability of the composite PEMs was adequate at least for 2 days, during which no degradation of the performance was observed. (C) 2009 Elsevier B.V. All rights reserved.