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Introduction In catalyst layers of polymer electrolyte membrane fuel cells (PEMFC), a solid polymer electrolyte, called “ionomer” to be distinguishable from the electrolyte membrane, is used to form electrochemical interfaces on catalyst surfaces. Recent studies, however, showed that a perfluorinated sulfonic acid polymer (e.g. Nafion), which is almost exclusively used for the ionomer, blocks active sites on Pt catalyst and, therefore, slows down the kinetics of oxygen reduction reaction (ORR) [1]. A plausible poisoning species is the sulfonic anion of the polymer molecule, the adsorption of which on Pt (111) surface is clearly seen in the cyclic voltammogram (CV) [1-3]. It is, therefore, expected to be effective for improving the ORR performance to use an ionomer with a functional group that is less adsorptive to Pt surface. Recently, Shinohara et al. developed a novel ionomer having two sulfonimide acid groups in its side chain ended with a hydrophobic group, instead of a sulfonic acid group, NBC4 (Fig. 1), which has higher protonic conductivity and O2 permeability than Nafion [4]. The new side chain likely has a stronger steric effect for anion adsorption, and NBC4 is expected to be lesspoisoning and have improved properties not only for the proton conduction and O2 transport but also for ORR kinetics. These expectations are examined experimentally in the present study.