A multiphase transient model is developed to investigate key physical and transport phenomena during the startup of a polymer electrolyte fuel cell (PEFC) from subzero temperatures. The proposed PEFC cold start model rigorously considers ice/frost formation and evolution in the cathode electrode, and their complicated interactions with heat transport, mass transport, and electrochemical reactions. The proposed cold start model is extensively validated against experimental data measured under various key cold start conditions such as startup temperature, startup current density, and initial water content in the membrane. The model predictions are in good agreement with the experimental data, demonstrating the validity and accuracy of the cold start model. In addition, detailed simulation results, including multidimensional contours, clearly elucidate the cold start behavior of a PEFC under different cell designs and operating conditions. We suggest that the proposed cold start model could aid in the development of optimal cold start strategies. (C) 2012 Elsevier Ltd. All rights reserved.