The effects of contaminants on the performance of an air breathing proton exchange membrane fuel cell (PEMFC) were investigated, by introduction into oxidant air fed to the fuel cell. The impact of the common pollutants sulphur dioxide, nitrogen dioxide, carbon monoxide, propane and benzene and the chemical warfare agents, sarin, sulphur mustard, cyanogen chloride (CNCl) and hydrogen cyanide (HCN) were assessed. At the concentrations studied, the common contaminants had either no effect on performance or caused a reversible depression. The chemical warfare agents all seriously compromised the performance of the fuel cells in an irreversible manner.