Methanol is considered as a good hydrogen carrier for mobile application of fuel cells. In such systems, the exhaust gas of the anode is a multicomponent gas composed of hydrogen and carbon dioxide as main constituents and methanol, carbon monoxide, and steam as minor species. It is necessary to burn out these gases in order to prevent emissions of combustibles and to increase the overall efficiency of the fuel cell. A study on the characteristics of a catalytic monolith reactor to oxidize such a fuel gas in near-adiabatic conditions is performed. Emissions and reactor wall temperature distributions are measured during the combustion of several fuel gas mixtures corresponding to design and off-design fuel cell conditions, in a large range of excess air ratios. The high surface reactivity and diffusion properties of hydrogen are seen to greatly enhance the combustion of the other species, by bringing the wall at temperatures sometimes greater than the expected adiabatic. Ultralow emissions of carbon monoxide and unburned hydrocarbons are achieved even at a temperature as low as 250 degrees C, allowing for great flexibility of the use of air or high anode hydrogen utilization for the fuel cell. In addition, startup and transient operations with excessive methanol content are shown to be well handled by the reactor. Finally, a practical engineering rule is given by defining two mass flow variables: one concerns the initial carbon containing fuel and the other the emitted unburned carbon containing fuel. These variables allow considering methanol and carbon monoxide as a single species, rendering the analysis independent of their respective concentrations in the multicomponent fuel mixture. A chart yielding three operational regimes (kinetic, diffusion, and optimal) can be defined which makes design for engineering purposes easier with regard to minimal pollutant emissions and optimal reactor power load.