The separate anodic and cathodic overpotentials in a proton exchange membrane fuel cell (activation, ohmic, concentration and mass transport) were measured in conventional and segmented hardware via reference electrodes, and multi-component gas analysis. The results show that the anodic overpotentials cannot be ignored, even when the operating conditions are changed at the cathode only. Under drying conditions the difference in the current density across the active area creates in-plane thermal gradients due to inhomogeneous reaction rates, and water phase changes. The resulting inhomogeneous temperature distributions can result in membrane and electrode degradation. The combination of reference electrodes and multi-component gas analysis enables the measurement and calculation of kinetic and diffusion parameters that can be used for modeling, and improved fuel cell design, efficiency, and durability. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.