This paper investigates the heat/mass transfer and electrochemical kinetics in the cathode catalyst layer of polymer electrolyte fuel cells (PEFCs) during cold start below the freezing temperature. A number of key parameters that govern the cold-start operations, such as time constants and the Damkohler number, are defined and their impacts are explored. A lumped parameter model for cell temperature and ice formation is developed and one-dimensional analysis is performed on the reactant transport in the catalyst layer. We found that a dimensionless parameter, defined as ratio of the time constant of cell warmup to that of ice-volume growth, is important for self-startup of fuel cells in subzero conditions, and a high value of tortuosity has profound impact on reactant starvation during cold start. In addition, the reduction in the electrochemical active surface of the catalyst layer due to the ice coverage is found to be a major mechanism leading to cell voltage loss during cold start. (c) 2007 The Electrochemical Society.