The use of hydrogen-deuterium (H-D) exchange as a method to introduce contrast in (1)H NMR microscopy images and to investigate the dynamic distribution of water throughout an operating H(2)/O(2) polymer electrolyte membrane fuel cell, PEMFC, is demonstrated. Cycling D(2)O(1) through the flow channels of a PEMFC causes H-D exchange with water in the PEM to result in a D(2)O-saturated PEM and thus concomitant removal of the (1)H NMR signal. Subsequent operation of the PEMFC with H(2)(g) enables visualization of the redistribution of water from wet or flooded conditions as H-D exchange occurs with D(2)O in the PEM and results in recovery of the (1)H NMR signal. Alternating between H(2)(g) and D(2)(g) as fuel allows observation of water distributions in the PEM while the cell is operating at a steady-state under low relative humidity. At similar currents, the rate of observable H-D exchange in the PEM during fuel cell operation was faster when the PEM was saturated with water than when under low relative humidity. These results are consistent with the known proportions of the conductive hydrophilic and nonconductive hydrophobic domains of Nation when exposed to different relative humidities. (C) 2007 Published by Elsevier B.V.