Understanding the dynamic behaviors of proton exchange membrane (PEM) fuel cells during startups is crucial for their proper operations to minimizing the detrimental effects of reverse current. In this study, the fuel cell is initially filled with nitrogen at both sides, and then reactants are supplied to the cell and the load is imposed simultaneously. Experimental results show that reverse current still occurs even though nitrogen purging is employed before the startup and hydrogen/air boundary is eliminated at the anode. The magnitude of reverse current density increases with the startup voltage. Analysis shows that the reverse current is proportional to the difference between the startup voltage and the open circuit voltage, and it occurs only when the open circuit voltage is lower than the startup voltage, which is due to the charging of the electrical double-layer on the cathode. Thus a startup strategy of air pre-filling is devised and the experimental results show that reverse current can be completely avoided or significantly reduced by pre-filling the cathode with air before startups. Another startup strategy, a linear startup mode, is also tested and the results show that it is effective in reducing the reverse current. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.