Gas diffusion electrodes for high temperature polymer electrolyte fuel cells (PEFCs) have been prepared by using a proton conductive borosiloxane (BSO) electrolyte. The catalyst layer was composed of Pt loaded carbon black (Pt-CB) and BSO ionomer. The polarization properties and the microstructure of the catalyst layer were investigated as a function of the composition. The anodic polarization was found to be negligibly small for all the compositions examined. The highest cathode performance was obtained at BSO/CB = 1.5 (by weight), where the best balance of high catalyst utilization and oxygen gas diffusion rate through the ionomer was obtained. (C) 2004 Elsevier B.V. All rights reserved.