In the study, a self-made kW-class 40-cell proton exchange membrane fuel cell (PEMFC) stack, with an active area of 112.85 cm(2) for each membrane electrode assembly and with the anodic Pt-Ru catalyst, was tested under different simulated reformate gases of different CO concentrations and different hydrogen concentrations. The performances and the transient voltages of the stack and the individual cells under different CO/N-2/H-2 mixtures were studied. The results show that increasing the CO concentration or decreasing the H-2 concentration of the CO-contained reformate gas negatively affects the performance of the PEMFC stack. Moreover, the PEMFC stack with the Pt-Ru anodic catalyst can tolerate a CO concentration of up to 50 ppm under non-diluted H-2. However, it can only tolerate 10 ppm CO under diluted H-2. The CO tolerance decreases dramatically with an increase in the H-2 dilution level. In addition, increasing the CO concentration in diluted H-2 or decreasing the H-2 concentration in CO-contained H-2 accelerates the occurrence of potential oscillation. The potential oscillation is owing to the interactions of CO electro-oxidation and adsorption reactions on the catalyst. This work is also the first to report that the potential oscillation phenomenon initially occurs at the upstream cells of the stack. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.