Starting PEFCs from sub-zero temperatures can be challenging as generated product water runs the risk to freeze, causing cell failure by blocking the gas pathways for the reactant gases in the porous layers. Hence a fundamental understanding of the cell failure mechanisms is essential to enable robust system functionalities even at temperatures far below 0 degrees C. In this work we set a focus on analysis of water transport processes during isothermal startup at temperatures between -10 and -2.5 degrees C. Neutron radiography was applied in order to analyze spatial heterogeneities of water production during a cold start and also to verify phase transitions from water to ice. The latter was facilitated by a recently developed dual spectrum neutron radiography method, which was applied for the first time to a 50 cm(2) test cell. Our results reveal that at -5 degrees C and above freezing can occur in a limited region while the rest of the cell continues generating liquid product water. But as temperature is shifted downwards, water distribution tends to be more uniform and freezing mechanisms seem to proceed more homogeneously over the cell plane. (C) 2016 The Electrochemical Society. All rights reserved.