The PBI/H3PO4 high temperature (HT) PEM fuel cell is often fueled with H 2-rich reformates which typically contain impurities such as CO, CO2 and water vapor. CO in the concentration level of several percent can cause apparent loss in performance of the HT-PEM fuel cell. In this study the influences of CO2 and water content in anode stream on CO poisoning of a HT-PEM fuel cell were investigated. Polarization curves and electrochemical impedance spectra were measured to quantify the performance loss caused by CO poisoning under different conditions. The experimental results suggest that higher anode humidity level reduced the cell performance loss caused by CO poisoning. When the fuel cell was operated with pure H2 , the cell performance was not significantly affect by the change in anode dew point temperature in the range of room temperature and 60 C. CO2 in anode stream resulted in slight decrease in performance of the fuel cell operated with pure H2 due to dilution effect. Whereas it can significantly deteriorate the cell performance loss caused by CO poisoning. Lastly, the CO poisoning effect on cell performance with presence of CO2 was proved to be relieved by anode humidification.