The distribution of carbon monoxide (CO) within a simple one dimensional polymer electrolyte fuel cell (PEFC) is studied experimentally using localised stripping voltammetry and adsorption transients. The effect of varying the carrier gas flow rate and CO dosage is investigated. It is found that residence time within the fuel cell is the key factor in determining the extent of poisoning or CO adsorption. Low flow rates are seen to result in a more anisotropic distribution of CO with greater amounts found away from the channel. High flow rates lead to a much more uniform profile. Diffusion of reactant into the gas distribution layer (GDL) and adsorption onto the catalyst retards the flow of reactant down the channel which broadens the peak width of the bulk adsorption transient. With knowledge of the catalyst roughness factor, pseudo 2-D reactant distribution profiles can be derived. These diagrams provide the equivalent of 'snap-shots' of the flow of reactants through the simple one dimensional fuel cell. This technique has applications in optimising the lateral distribution of catalyst and MEA properties such as GDL porosity. (C) 2004 Elsevier B.V. All rights reserved.