The influence of the ionomer content on proton transport phenomena in the catalyst layers of a fuel cell was examined using pseudo catalyst layers (PCLs) consisting of the carbon support and the ionomer. Effective proton conductivity sigma(eff) of the PCL increased with increasing ionomer content in both graphitized ketjen black (GKB) and ketjen black (KB) systems. It was found that the ionomer coverage theta and the Bruggeman factor gamma, which is related to the tortuosity of the proton path in the PCL, did not depend on the ionomer content appreciably. In the GKB system, the volume fraction of the ionomer epsilon(ion) was the dominant factor of the difference in sigma(eff). However, the results of a water adsorption analysis with a heterogeneous Do-Do model suggested that the proton conductivity of the ionomer as bulk sigma(bulk) probably differs depending on the ionomer content in the KB system, though it was independent of the ionomer content in the GKB system. It was inferred that the interaction between functional groups on the KB surface and the ionomer weaken its water adsorbability. Consequently, the ionomer content would appear to affect not only structural parameters (i.e. gamma and epsilon(ion)) but also sigma(bulk) in the KB system. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3598141] All rights reserved.