The electrode ionomer is a key component of the catalyst layer and therefore the introduction of new electrode ionomer would have a significant effect on cell performance. To investigate the effect of sulfonated poly(ether ether ketone) (sPEEK) as the electrode ionomer, the catalyst layer was prepared using sPEEK ionomer and its physical properties such as pore structure and the hydrophobicity were examined. The electrochemical characteristics of the membrane and electrode assemblies employing sPEEK ionomer-based catalyst layer were also analyzed. Even though sPEEK ionomer-based catalyst layer showed lower ohmic resistance compared with Nafion ionomer-based catalyst layer, complete pore-filling of sPEEK into the primary pore caused reduction in reactive surface area of the catalyst and hindrance in gas transfer. In addition, the very low hydrophobicity of sPEEK ionomer-based catalyst layer deteriorated gas transfer due to an increased water flooding. (c) 2009 Elsevier Ltd. All rights reserved.