The use of reformed fuels in polymer electrolyte fuel cells (PEFC) exhibits distinctive advantages over pure hydrogen in terms of fuel supply infrastructure and on-board fuel storage in a vehicle. Yet small residual CO in the reformate in the order of 10-100 ppm cause severe degradation of fuel cell performance. The CO tolerance of the fuel cell is therefore an important characteristic calling for adequate quantitative evaluation. In this study, we assessed a range of methods to quantify the CO tolerance based on cell polarization data. Generally, no method has the claim to be ideal, but the appropriate choice of CO tolerance quantity rather depends on the design target of the fuel cell system. Analysis of literature data revealed, however, that comparison of fuel cell performance using pure and CO-contaminated hydrogen, respectively, at constant current density seems to be a popular method, because it yields immediate evidence of anode performance. It was also recognized that CO tolerance manifests on different levels: on the electrocatalyst, the electrode, and the fuel cell level. In order to investigate the CO tolerance on the electrode level, a fuel cell of one-dimensional construction, like the one used in our experiments, needs to be employed.