A novel experimental method for the measurement of mass transport losses in a polymer electrolyte fuel cell (PEFC) was developed, based on the comparison of cell voltage during operation with air, helox, (79% He, 21% O-2) or pure O-2. Using only short periods (2 seconds) of helox or pure O-2 operation, the disturbing artifacts (dry out or change of catalyst surface oxidation state) associated with continuous operation with these gases were avoided. High resolution neutron imaging was conducted simultaneously in order to study the relation between mass transport loss and liquid water accumulation. For the studied design, comparisons in steady state and dynamic experiments indicated a larger impact of water accumulation in the gas diffusion layers (GDL) under the flow channels, or in the portion of the channels near the GDL surface. Increased diffusion losses were observed, as expected, under high humidity conditions, but also at very low humidity conditions in absence of any liquid water. The latter was explained by a redistribution of current toward the rib region, having longer diffusion paths. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.017207jes] All rights reserved.