The structure, composition, and interfaces of membrane electrode assemblies (MEA) and gas-diffusion layers (GDLs) have a significant effect on the performance of single-proton-exchange-membrane (PEM) fuel cells operated isothermally at subfreezing temperatures. During isothermal constant-current operation at subfreezing temperatures, water forming at the cathode initially hydrates the membrane, then forms ice in the catalyst layer and/or GDL. This ice formation results in a gradual decay in voltage. High-frequency resistance initially decreases due to an increase in membrane water content and then increases over time as the contact resistance increases. The water/ice holding capacity of a fuel cell decreases with decreasing subfreezing temperature (-10 degrees C vs. -20 degrees C vs. -30 degrees C) and increasing current density (0.02 A cm(-2) vs. 0.04 A cm(-2)). Ice formation monitored using in-situ high-resolution neutron radiography indicated that the ice was concentrated near the cathode catalyst layer at low operating temperatures (approximate to-20 degrees C) and high current densities (0.04 A cm(-2)). Significant ice formation was also observed in the GDLs at higher subfreezing temperatures (approximate to-10 degrees C) and lower current densities (0.02 A cm(-2)). These results are in good agreement with the long-term durability observations that show more severe degradation at lower temperatures (-20 degrees C and -30 degrees C). (C) The Author(s) 2016. Published by ECS. All rights reserved.