Abstract: A process for controlling the hydrogen that evolves as a parasitic by-product during the operation of a metal/air fuel cell which comprises feeding into a catalytic recombiner a gaseous stream containing hydrogen being discharged from the metal/air cell together with an amount of oxygen sufficient to convert the hydrogen to water, said oxygen gas concentration being monitored and adjusted to maintain an oxygen concentration in the gaseous stream entering the recombiner in the range of 1.5 to 2.5% by volume. The process is of particular value in closed system aluminum/oxygen fuel cells in which small amounts of hydrogen must be removed, e.g. the sealed hull of an underwater vehicle.