This paper presents a method for collecting ac impedance data from a commercial PEFC power system at operational loads. The PEM fuel cell stack in the power system, including 47 MEAs, was operated using room air and pure hydrogen (>99.99%). For a stack test in the power system, the power source for the embedded controller board was simultaneously switched from the fuel cell stack to a similar external power source after the system reached a steady temperature. The PEM fuel cells in the stack were tested by collecting the ac impedance data at different current levels. By using ac impedance, a single fuel cell, a group of fuel cells, and a complete stack were then tested without the embedded control devices for ohmic, activation, and mass transport losses. The ohmic resistance for each cell component in the stack was obtained as It 7 m Omega cm(2) at operational loads from 2.5 A to 35 A. The membrane thickness was further estimated as ca. 51-89 mu m. Resistances from ohmic conduction, anode/cathode activation, and mass transport were measured and discussed using the Nyquist plots from the ac impedance spectra. (C) 2007 Elsevier B.V. All rights reserved.