Gas permeation through a membrane electrode assembly (MEA) is an important issue in the development of polymer electrolyte membrane (PEM) water electrolyzers, because it can cause explosions and efficiency losses. The influence of operating pressure, temperature and MEA modifications on the permeation was already investigated. However, most of the studies pay no attention to the compression of the porous transport layer (PTL) of the MEA when assembling it in a test cell to carry out the experiments. This paper deals with the impact of the PTL compression on hydrogen permeation and cell voltage. Polarization, impedance and permeation measurements are used to demonstrate that the compression significantly affects the MEA's properties. Measurements show either a linear or nonlinear correlation between current density and hydrogen permeation, depending on the compression. The results indicate that the compression of the PTL must be taken into account for developing MEAs and comparing different permeation measurements. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.