The expansion of Nafion by water uptake poses a practical problem for the assembly of a fuel cell. In order to determine an appropriate assembling procedure, a study is presented of the expansion of a standard membrane (dispersion cast NRE-212) by water absorption under compression, and its influence on the response of a fuel cell. The expansion of uncompressed NRE-212 membranes determined by optical microscopy is 9.4% in plane and 30% in thickness, after soaking in water at room temperature. No significant difference is observed in the expansion of the membrane covered with a catalyst layer (catalyst coated membrane, CCM). Cross-sectional imaging of membrane-electrode assemblies (MEAs) shows that under compression between plates mimicking the gas flow field of a cell, the membrane undulates with micrometric amplitude (similar to 100 mu m), following ribs and channels of the plates, whereas the gas diffusion layer (carbon cloth) deforms and enters the channel covering 25% of its 1 mm(2) section. No significant differences in the cross-section profile are observed if mounting a dry or a previously humidified, expanded, Nafion membrane, neither by submitting it to dry-humidification cycles. However, single cell response shows important differences between cells assembled with dry membranes and cells assembled with previously expanded membranes. Polarization curves show that the assembly with an expanded membrane gives rise to lower internal resistance, allowing for an improvement in the maximum power density of about 7% at 50 degrees C and about 13% at 80 degrees C, under standard testing conditions. Impedance spectroscopy shows that the improved response of the pre-expanded membrane must be attributed entirely to better ionic conductivity, especially noticeable when measured at high temperature (80 degrees C). On the other hand, some increase in catalyst layer resistance is registered with the pre expanded membrane that we attribute to a reduction in the catalyst surface density as a consequence of the expansion. This effect is less important than the ionic resistance effect for the response of the cell. From a practical point of view, the assembly with wet, fully expanded, membranes performs better at high current densities (>0.2 A cm(-2)). At lower current densities, the assembly with a dry membrane performs like the expanded one. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.