Spatial, quantitative, and temporal information regarding the water content distribution in the transverse-plane between the catalyst layers of an operating polymer-electrolyte membrane fuel cell (PEMFC) is essential to develop a fundamental understanding of water dynamics in these systems. We report H-1 micro-magnetic resonance imaging (MRI) experiments that measure the number of water molecules per SO3H group, lambda, within a Nafion (R)-117 membrane between the catalyst stamps of a membrane-electrode assembly, MEA. The measurements were made both ex situ, and inside a PEMFC operating on hydrogen and oxygen. The observed H-1 MRI T-2 relaxation time of water in the PEM was measured for several known values of A. The signal intensity of the images was then corrected for T-2 weighting to yield proton density-weighted images, thereby establishing a calibration curve that correlates the H-1 MRI density-weighted signal with lambda. Subsequently, the calibration curve was used with proton density weighted (i.e.. T-2-corrected) signal intensities of transverse-plane H-1 MRI images of water in the PEM between the catalyst stamps of an operating PEMFC to determine lambda under various operational conditions. For example, the steady state, transverse-plane lambda was 9 +/- 1 for a PEMFC operating at similar to 26.4 mW cm(-2) (similar to 20.0 mA, similar to 0.661 V, 20 degrees C, flow rates of the dry H-2(g) and O-2(g) were 5.0 and 2.5 mL min(-1), respectively). (C) 2010 Elsevier B.V. All rights reserved.