Knowledge of the species distribution within a polymer electrolyte fuel cell (PEFC) is critical to fully characterize the local performance and accurately quantify the various modes of water transport. This paper describes the results of a newly developed real-time technique for in situ measurement of water vapor, nitrogen and oxygen distributions within the reactant flow channels of an operating PEFC using an Agilent real-time gas analyzer (RTGA). The RTGA was interfaced directly to the fuel cell at various locations along the single path serpentine anode and cathode flow channels of a specially designed fuel cell. Species distribution data are combined with measured high frequency resistance and current distribution. The periodic presence of liquid water droplets in the cathode at high current density and at locations near the cathode exit is also observed. (C) 2004 Elsevier B.V. All rights reserved.