A physical model for PEM fuel cell impedance Z at open circuit potential (OCP) is developed and analytical expression for Z is derived. The OCP impedance is a sum of the cathode catalyst layer (CCL) and the gas-diffusion layer (GDL) impedances connected in series. The GDL impedance differs from the Warburg impedance, which is often used in modeling of fuel cell electrodes. A key parameter determining the OCP impedance spectrum is the Newman's dimensionless reaction penetration depth epsilon. In PEMFCs, epsilon is large, which makes the GDL impedance a small "invisible" additive to the CCL impedance. In a solid oxide fuel cell (SOFC) anode, epsilon is small, and the GDL impedance forms a well-resolved separate arc in the impedance spectrum at the OCP. In a real PEM fuel cell, a true OCP regime cannot be achieved due to hydrogen crossover through the membrane. Impedance measurements at zero current in the load yield an equivalent current density of hydrogen crossover through the membrane. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.