In this work, we investigate the cold-start operation of polymer electrolyte fuel cells (PEFCs) through high-resolution neutron radiography, experimental testing, theoretical evaluation, and comparison with model prediction. Ice formation location, voltage evolution, and loss of the electro-catalyst surface area (ECSA) are examined. A dimensionless parameter (h) over bar, characterizing the spatial variation of the reaction rate across the cathode catalyst layer, is discussed at subfreezing temperature using newly determined membrane ionic conductivity. The (h) over bar evaluation identifies the operating range that the reaction rate can be treated uniform across the catalyst layer, in which the model is valid. (C) 2012 Elsevier Ltd. All rights reserved.