An in situ technique for analyzing ionomer coverage, theta(ion), in catalyst layers was examined. Contributions of Pt and water to electric double layer capacitance, C-dl, were extracted by carbon monoxide (CO) adsorption and the RH dependence of C-dl, respectively. Consequently, theta(ion) for Pt (theta(ion,pt)) and the carbon support (theta(ion,C)) were obtained by comparing C-dl attributed to different interfaces, namely, the interface between the electroconductor and the ionomer or that between the electroconductor and water. The validity of the method was verified by various analyses and experiments, such as water adsorption analysis, analysis of specific capacitance and electrochemical surface area analysis by CO stripping voltammetry. theta(ion) of the catalyst layer with two different carbon supports, either ketjen black (KB) or graphitized ketjen black (GKB), was evaluated with this method. In the case of a GKB-based catalyst layer, CO apparently did not cover the entire Pt surface, making the analysis more complicated. In this case, applying C-dl of a pseudo catalyst layer, consisting of the carbon support and the ionomer, was deemed as appropriate. The analysis revealed that theta(ion,Pt) and theta(ion,C) were ca. 0.39 and ca. 0.25 in the KB system and ca. 0.49 and more than 0.91 in the GKB system. Although the number of primary pores in GKB is very small, theta(ion,Pt) seemed small compared with theta(ion,C). From SEM and TEM images, it was found that Pt particles formed clusters on the GKB surface. Presumably, the narrow voids of the Pt clusters contributed to the low theta(ion,Pt). (C) 2013 Elsevier B.V. All rights reserved.