The present invention provides a method for selective production of hydrogen and methane from a biomass feedstock in a reactor vessel maintained under anaerobic conditions by controlling moisture concentration in the reactor vessel. The method comprises the steps of introducing a biomass feedstock into the reactor vessel maintained under anaerobic conditions; introducing a bacterial inoculum into the reactor vessel to facilitate digestion of the biomass feedstock; setting a moisture concentration of the contents of the reactor vessel at a first moisture level; and collecting hydrogen gas from the reactor vessel. The method may further comprise maintaining the moisture concentration at a first moisture level or within a first moisture range. The method may additionally comprise increasing the moisture concentration of the contents of the reactor vessel to a second moisture concentration; and collecting methane gas from the reactor vessel. Switching between hydrogen and methane production is controlled by adjusting the moisture concentration of the contents of the reactor vessel.