Ohmic losses in fuel cells contain both ionic (mainly protonic) and electronic contributions. In this work, we developed a method to distinguish the resistance contributions from individual components. Proton transport resistance of proton exchange membranes (PEMs) in the through-plane direction, the major proton transport direction in in situ fuel cells, was measured using electrochemical impedance spectroscopy combined with compression-controlled fuel cell hardware. Membrane resistance was obtained by subtracting the nonmembrane contributions from the total resistance. Proton conductivities of PEMs with different equivalent weights, calculated from through-plane resistance measurements at various relative humidity conditions, were compared with good agreement to the in-plane measurements. Fuel cell electronic resistance and membrane-electrode interfacial resistance were also evaluated using both ex situ and in situ methods. The membrane-electrode interface resistance was nearly constant over a range of relative humidity conditions. The effects of test procedure and cell build strategy were investigated and had a significant impact on the membrane resistance measurements. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3240877] All rights reserved.