A transient physical model of the cathode catalyst layer (CCL) in a PEM fuel cell is linearized and an exact analytical solution for the CCL impedance (Z) is derived. The solution is valid at a low current density, when the proton and oxygen transport losses are not large. The zero-frequency resistance contains the terms due the proton and oxygen transport. The analytical equation for Z is fast enough for accurate experimental spectra fitting, which yields the ORR Tafel slope, the CCL proton conductivity and oxygen diffusivity. (C) 2014 Elsevier Ltd. All rights reserved.