Ionic resistance and double layer capacitance of catalyst layer (CL) variations with the thickness of the cathode CL were estimated by electrochemical impedance spectroscopy (EIS) using the modified transmission-line model (TLM). 0.1-0.4 mg(Pt) cm(-2) of commercial 20, 40, and 60 wt% Pt/C catalysts were used to control cathode CL thicknesses. The catalyst with a low Pt to C ratio was favorable for maximum power density when the catalyst loaded was <= 0.2 mg(Pt) cm(-2), and the catalyst with a high Pt to C ratio was favorable when catalyst loading was >= 0.3 mg(Pt) cm(-2). The electrochemical surface areas of 20, 40, and 60 wt% Pt/C were different, whereas the Pt utilization of these catalysts was similar. Modified TLM under the non-faradaic condition was used for an easy estimate of the ionic resistance and double layer capacitance of the CL. Moreover cell inductance, electrolyte resistance, and the charge transfer resistance of crossovered H-2 were used for a precise estimate. The ionic resistances and double layer capacitances of the CL were linearly proportional to CL thickness regardless of the Pt to C ratio. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.030204jes] All rights reserved.