Dehydration phenomena in a PEM fuel cell were investigated by nonlinear frequency response analysis (NFRA) in a differential H-2/H-2 cell. The linear H-1,H-0 spectra, which are equal to classic EIS spectra, showed not only an increase of the membrane resistance but also an increase of the anode reaction resistance, caused by dehydration leading to the decrease of the protonic conductivity of the polymer network in the catalyst layer. With this, active sites with long protonic pathes to the membrane become inactive. In order to further clarify this effect, modelling work was used. Therefore, proton transport was incorporated into an existing model of a differential H-2/H-2 cell. Finally, the key features of NFRA spectra under dehydration and CO poisoning are compared in order to discuss the suitability of NFRA for unambiguous diagnosis of PEMFC. It can be seen that while the linear spectrum is not sufficient to distinguish between both cases, the second order frequency response functions can be used for discrimination. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.