The oxygen transport resistances of Nafion thin films on a Pt electrode and of a bulk Nafion were evaluated to clarify the dominant part of the oxygen transport loss in an ionomer for a cathode catalyst layer of polymer electrolyte fuel cells. To evaluate the oxygen transport resistances of the Nafion thin film, an oxygen flux through the Nafion thin film on the Pt was detected as diffusion limited current density, j(d), for an oxygen reduction reaction by using an electrochemical cell equipped with a Pt microelectrode coated with a Nafion film less than 100 nm. An interfacial oxygen transport resistance and inner resistivity of the Nafion thin film were separately determined by the relation between j(d)(-1) normalized by oxygen partial pressure and the film thickness. The interfacial resistance is equivalent to 30-70 nm of the Nafion film, which are much thicker than a typical ionomer thickness of similar to 10 nm observed in catalyst layers. From the above results and our recent theoretical results obtained by molecular dynamics techniques, the oxygen permeation fluxes through the ionomers in catalyst layers can be regarded as dominantly controlled by the oxygen permeation at the Pt/ionomer interface. (C) 2016 Elsevier Ltd. All rights reserved.