Initially measured cell voltages during isothermal, galvanostatic operation at -20 degrees C were analyzed. Individual contributions from various known voltage loss sources, including electrode resistances, were either measured a priori or modeled in order to quantify. Similar to nonfrozen conditions, voltage losses due to the sluggish kinetics of oxygen reduction reaction persist at subfreezing temperatures, contributing a majority of the overall losses. Membrane and cathode catalyst layer resistances also contribute a significant portion, due to decreased proton conductivity at subfreezing temperature. Catalyst utilization modeling within the cathode electrode thickness indicates that oxygen reduction reaction favors the electrode/membrane interface at moderate current densities. Cryo-scanning electron microscope (SEM) images confirm this prediction, which indicates that filling the electrode with ice occurs from the membrane outwards toward the diffusion medium. At lower currents, where the model predicts more uniform catalyst utilization across the thickness, more even filling of the electrode appears from cryo-SEM images. Due to the sensitive time-dependency of anode and cathode catalyst layer resistances, as well as membrane resistance, direct In Situ measurement of these parameters to elucidate voltage losses during operation of polymer electrolyte membrane fuel cells at subfreezing temperatures is presently difficult, if not impossible. (C) 2008 The Electrochemical Society.