The water produced in a proton exchange fuel cell can exist in supercooled states during cold start operation. Visualization studies of unit cells under cold start conditions have confirmed that supercooled states exist in the cells and that they are eventually released. However, these supercooled states have not been quantitatively characterized, and it is difficult to predict them. Moreover, it has not been clarified what determines the supercooling degree and the release of supercooled states in each part of the cells, such as the gas flow channels, gas diffusion layers, and catalyst layer. In this work, a theoretical model was developed to predict the release of supercooled states on the basis of heterogeneous nucleation theory and by considering the surface wettability of the porous media in the cells. This model was evaluated through comparison to our in situ visualization study of a cell during a cold start. The developed model reproduced the supercooled state in the cell well, specifically its release time, and quantitatively clarified the impact of the pore diameter and wettability on the supercooled states. (C) 2014 Elsevier Ltd. All rights reserved.