NOVELTY - Carbon monoxide in a fuel cell reformate feed stream is monitored using a mini fuel cell probe located in the stream which is discharged at intervals to measure the carbon monoxide concentration and purged at intervals to remove carbon monoxide from the probe anode catalyst.    USE - In monitoring and controlling CO concentration at locations in a PEM fuel cell stack used for e.g. electric vehicles.    ADVANTAGE - The method allows real time monitoring of CO concentration in the reformate feed stream with repeated refreshment of the probe anode. The method is useful during startup to determine whether the CO level in the effluent from the preferential oxidation (PROX) reaction is sufficiently low for direct use in the fuel cell; and is also useful for real time control of the oxygen supply to the PROX reaction.    DETAILED DESCRIPTION - Carbon monoxide concentration in the reformate feed stream of a proton exchange membrane (PEM) fuel cell is monitored by:    (1) providing a CO sensor comprising a PEM probe having anode and cathode surfaces, the anode having a catalyst which is susceptible to CO degradation over time;    (2) contacting the probe anode with the reformate stream;    (3) contacting the cathode with oxygen;    (4) discharging the probe periodically and measuring its output;    (5) comparing the output to that of a reference probe at known CO concentrations to determine the reformate stream CO concentration indicated by the probe; and    (6) periodically purging the anode catalyst of CO to maintain its performance.    In a modified method, the fuel cell is a hydrogen-oxygen fuel cell stack, and the CO monitoring method is used to control the rate of addition of air to the H2 fuel stream to optimize CO consumption from the stream with minimum H2 consumption.    DESCRIPTION OF DRAWING(S) - The drawing shows a bipolar PEM fuel cell stack with a sensor and data processor for CO monitoring and control of oxygen supply to the PROX reaction in accordance with the method of the invention    Fuel cell stack (2)    CO sensor (40)    PEM probe (41)    Discharge circuit (D)    Purge circuit (P)    Motorized switch for connection between discharge and purge circuits (80)