In this contribution the reversal line, a new voltage-current (U-i) characteristic curve, is proposed to analyze the transient behavior after an arbitrary load step. The response of a differential polymer electrolyte membrane (PEM) fuel cell to the changes in current, voltage, resistance and power is evaluated, and the existence of the curve is established. Furthermore, the influence of the starting point and the relative humidity is investigated. By using this approach, it is possible to predict the trajectory followed after any load step in the U-i plane and to anticipate infeasible load steps as well as the presence and magnitude of overshoots or undershoots in the cell response. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.