The volume sensitive residence time distribution method is ideally suited for the study of liquid water and ice formation within operating proton exchange membrane fuel cells. Sensitivity was demonstrated with the use of simulated water drops within the flow field channel (machined obstructions) yielding a linear correlation in the 0-20% volume obstruction range between measured and theoretical hydraulic volumes. The correlation was independent of obstruction spatial distribution but dependent on gas flow rate. Sensitivity was also demonstrated by varying the amount of liquid water within a gas diffusion electrode resulting in a linear correlation in the 7-44% void volume obstruction range between normalized time difference between the points at which the tracer concentration has decayed by 20 and 90% of the steady-state value prior to the tracer injection interruption and measured gas diffusion electrode liquid water content. Sensitivity to liquid water obstructions was maintained using an operating fuel cell and two different gas diffusion media with relatively similar transport properties but further work is needed to separate flow field from gas diffusion electrode contributions. The usefulness of the residence time distribution is also demonstrated for other applications, including gas crossover through the proton exchange membrane, flow distribution uniformity and gas diffusion electrode compressibility/deformation. (c) 2006 Elsevier B.V. All rights reserved.