For further improvements in the startup ability below freezing and the durability of polymer electrolyte fuel cells, understanding the ice formation mechanism during cold startup is particularly significant. This study observes cross-sectional ice distributions in a catalyst layer (CL) during isothermal galvanostatic operation at -20 degrees C using a cryo-scanning electron microscope. The effects of current density, cathode gas conditions, initial water content of the membrane, and cell temperature on the cold start characteristics and the ice formation process in the CL are evaluated. The observational results show that at higher current densities, the region with active ice formation moves from the membrane to the gas diffusion layer sides during the freezing period and vacant pores remain near the membrane even after cell shutdown, while the pores are completely filled with nearly-uniformly growing ice at lower current density operation. This is consistent with the experimental finding from the cold start characteristics that the estimated amount of ice accumulated in the cell until the shutdown decreases as the current density increases. Contrary to expectations, these changes are largely independent of cathode gas conditions, even with pure oxygen. Additional factors controlling the ice formation process are discussed based on the experimental results. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.