Time-averaged cell voltage, cell efficiency, and power density in an autonomous oscillatory state was higher than that in stable steady-state for a proton exchange membrane fuel cell (PEMFC) operating in the presence of CO in the anode feed reformate gas. The average power density under such an oscillatory state at 55 degreesC is twice that under a stable steady state. The average cell voltage and power density gain are due to the decrease of the time-averaged anode overpotential in the oscillatory state. This report thus provides experimental evidence that autonomous oscillatory operation of fuel cells in the presence of CO can increase the power output as compared to that in stable steady-state operation. (C) 2004 The Electrochemical Society.