In this study, experimental constant-current cold starts were performed on a polymer electrolyte fuel cell from -10 degrees C to characterize high-frequency resistance behavior, water motion, and ice accumulation before, during, and after cold start. A diagnostic method for rapid and repeatable cold starts was developed and verified. Cold-start performance is found to be optimized when cell resistance is increasing prior to startup, which is indicative of polymer electrolyte membrane (PEM) dehydration. During cold start, cell resistance initially decreases due to PEM hydration by the product water. Interestingly, after a certain water-uptake capacity of the PEM is reached, resistance increases due to ice formation in and around the cathode catalyst layer (CL), with some evidence of supercooled water flow at low currents. Utilizing lower startup currents apparently does not increase the PEM water-storage capability but does increase the total volume of ice formation in and around the CL. Lower startup currents were found to produce more total heat but at a reduced rate compared to high currents. Therefore, an acceptable current range exists for a given stack design which balances the total heat generation and time required to achieve a successful cold start. (c) 2008 The Electrochemical Society.