A characterization of the ionic conduction of the active layer of a polymer electrolyte membrane fuel cell (PEMFC) cathode by ac impedance measurement at open-circuit potential conditions was conducted. Porous electrode theory was used to derive a compact equation, partial derivative(2)(&UPhi;) over cap (2)/partial derivativey(2) + partial derivative ln f(y)/partial derivativey x partial derivative(&UPhi;) over cap (2)/partial derivativey - R/f (y)(1 + jOmega)(&UPhi;) over cap (2) = 0, to solve for the impedance response of a cathode at open-circuit potential conditions. This equation includes a parameter R, the ratio of an ionic resistance (evaluated at the active layer/membrane interface), to the total charge-transfer resistance of the active layer. The influence of an assumed ionic conductivity distribution profile f ( y) on the error in the estimation of total double-layer capacitance of the active layer from the -1/(Z(Im)omega) vs. Z(Re) plot was also investigated in this work. The increase of ionic conductivity in the active layer of an air cathode with an increase in the ionomer loading was revealed from both impedance data and surface area measurements. A nonlinear parameter estimation method was used to extract the ionic resistance from the high-frequency region of the impedance data at open-circuit potential conditions. The assumed ionic conductivity distribution profile in the active layer was found to vary with ionomer loadings. (C) 2003 The Electrochemical Society.