NOVELTY - Hydrogen-oxygen fuel system is operated by providing a reformate gas having a carbon monoxide content, and subjecting the carbon monoxide to an oxidation reaction with oxygen to produce a feed stream for the fuel cell. A series of pulses of oxygen-containing gas are generated and injected into the reformate gas, while the pulse duration and/or interval between pulses is varied.    USE - For existing fuel cell systems and process controllers.    ADVANTAGE - Enhances oxidation of CO to carbon dioxide, while minimizing excessive oxidation of hydrogen to hydrogen oxide. The method thus, enhances operation of the fuel cell system.    DETAILED DESCRIPTION - Operation of the hydrogen-oxygen fuel system involves providing a hydrogen-rich reformate gas having a carbon monoxide content, and subjecting the reformate gas to an oxidation reaction of CO with oxygen to produce a feed stream for the fuel cell. The feed stream has a CO content less than that of the reformate gas. A series of pulses of oxygen-containing gas are generated and injected into the reformate gas, while the pulse duration and/or interval between pulses is varied. The step is for optimizing consumption of CO from the fuel gas while minimizing consumption of hydrogen to produce a second carbon content less than that of the reformate gas. An INDEPENDENT CLAIM is also included for a fuel cell system including a source of a stream comprising hydrogen and carbon monoxide, a reactor, an injection system, an electronic control module (ECM), and fuel cell(s). The reactor is for oxidizing CO. It has a chamber with inlet and outlet connected with the stream and a catalyst dispersed and supported on a carrier. The catalyst is arranged to contact the stream passing between the inlet and outlet. The injection system has injector(s) (8) to supply controlled pulses of gaseous mixture comprising oxygen into the stream. Oxygen preferentially catalyze oxidation of CO to produce a CO depleted stream. The ECM energizes the injector and establishes duration of pulses and interval between pulses. The fuel cell consumes the CO depleted stream in the production of electrical energy.    DESCRIPTION OF DRAWING(S) - The drawing shows an cutaway elevational view of an injector.    injector (8)