Proton Exchange Membrane (PEM) Fuel Cell is a key component for the exploitation of hydrogen energy. Its diagnosis relies on several in-situ diagnosis tools. Electrochemical impedance spectroscopy is a major one. But this technic has some drawbacks, among which the complexity of the required equipment and the measurement time. An alternative method for diagnosis is developed in this work. A time-constant spectrum (or relaxation-time distribution) is extracted from the fuel cell voltage response to small current steps. This spectrum can be read as a distribution of series RC cell representing, such as an impedance spectrum, the whole dynamic of the voltage response to a given current excitation. In order to demonstrate the ability of the method to diagnose flooding, different simulations of voltage responses are first performed using appropriated parameters' variations in a fuel cell model. The resulting time-constant spectrums are analyzed to underline the sensitivity to the flooding effect. Then, experiments are achieved to confirm the potentiality of this technic to diagnose flooding. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.