During the clamping of a Fuel Cell, the porosity of the gas diffusion layer (GDL) and the contact resistances are continuously reduced. In this work, the optimum clamping pressure is determined for three commonly used GDLs. For this purpose, polarization curves are recorded for contact pressures of 0.1-2.7 N/mm(2). For the SGL 29BCE material, the optimum contact pressure range is very narrow, while the Toray TGP-H 060 material performs best in a wider range; the Freudenberg H2315 material is the most robust type, and works best from 0.6 N/mm(2) until the maximum (2.7 N/mm(2)). For the understanding of the mechanical effects on the GDL structure, scanning electron microscopy and nano- computer tomography images are made. In addition, permeability measurements are carried out. To transfer the results, the local pressure distributions are recorded, and their minima and maxima are considered. Ultimately, this information allows the transfer of the results to other flow field geometries. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.