In this work, the galvanostatic step sweep was used to investigate the hysteresis phenomenon found in the polarization measurements in proton exchange membrane (PEM) fuel cells. Three different diffusion media including Toray, E-TEK and SGL as well as the PTFE content from 10% to 30% in the diffusion media were carefully examined. Based on these results along with the comparison between results using dry and fully humidified air, the reason for the formation of such hysteresis in polarization curves was clarified. The generation and storage of liquid water was correlated with the materials of diffusion layers as well as the cross point in polarization curves during the forward and backward sweeps. Experimental results on the effect of operation conditions, i.e. flow rates, cell temperature and the time at each current density were also explored to illuminate the transient behavior within fuel cells behind such hysteresis phenomenon. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.