Proton conduction resistance in the cathode electrode with various ionomer/carbon weight ratios (I/C-ratios) was characterized using AC impedance at different relative humidities (RH) in a H2/N2 cell. AC impedance data were analyzed by fitting sheet resistance, Rsheet, sheet capacitance, Csheet, and the high-frequency resistance, HFR, to a simplified transmission-line model with the assumption that the proton resistance and the pseudo-capacitance are distributed uniformly throughout the electrode. For given types of model membrane electrode assemblies (MEAs), a percolation threshold for proton conductivity in the electrode was observed at I/C-ratios of approximately <0.6/1. The comparison to H2/O2 fuel cell performance shows that the ohmic loss in the catalyst layer can be quantified by this technique, and the parameter Rsheet extracted from the AC impedance spectra is indeed an intrinsic RH-dependent electrode property.