This paper describes the results of adding small amounts of CO gas to the cathode side in a HT-PEM fuel cell with a polybenzimidazole (PBI) membrane running on either oxygen or air. Experimental conditions: Temperature ranges 120-160 degrees C, constant current either 200 mA/cm(2) or 800 mA/cm(2) and CO ranges 0.1-1.3%. In this case it was found that small amounts of CO under special conditions have a beneficial effect on the potential of the fuel cells, whereas larger amounts can bring the potential down to almost zero. An interesting phenomenon is that after the flow of CO is switched off a temporary improvement of the potential is seen before the situation goes back to normal. A good explanation for this is a competition between CO, O-2 and H3PO4 at the three phase boundaries, also that a steady state exist in which CO constantly is oxidized to CO2. (C) 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.