Proton transport resistance in the polymer electrolyte of proton exchange membrane fuel cells (PEMFCs) is a significant contributor to ohmic overpotential. Conductivity is largely controlled by water uptake, which requires a delicate water balance to avoid an increase in mass transport overpotential due to diffusion resistance in porous components. The present work explores this relationship with distributed liquid water and high frequency resistance (HFR) measurements. The HFR response to local changes in liquid water content is characterized for two gas diffusion layers (GDLs) with different thermal resistivities. From these experiments, it is shown that liquid water must be present in the gas diffusion layer (GDL) to minimize the HER. Furthermore, the impact of varied saturation levels induced by GDL thermal gradients on drying transients is considered with both in-plane and through-plane neutron imaging. (C) 2012 Elsevier B.V. All rights reserved.