NOVELTY - Selective production of hydrogen and methane from a biomass feedstock in a reactor vessel maintained under anaerobic conditions, involves: (a) introducing a biomass feedstock into a reactor vessel maintained under anaerobic conditions; (b) introducing a bacterial inoculum into the reactor vessel to facilitate digestion of the biomass feedstock; (c) setting a moisture concentration of contents of the reactor vessel at a first moisture level; and (d) collecting hydrogen gas from the reactor vessel.    USE - For selective production of hydrogen and methane from a biomass feedstock in a reactor vessel maintained under anaerobic conditions. The biomass feedstock is selected from plant material, animal material, food water, industrial waste, and organic waste products or its residual. The biomass feedstock comprises cellulose (all claimed).    ADVANTAGE - The method is more cost-efficient; and produces hydrogen and methane simultaneously or separately. The method does not require additional pretreatment steps (e.g. pretreatment with heat (such as dry heat or wet heat) or a chemical agent (such as acids, antibiotics and methanogen inhibitor)) to enrich the bacterial inoculum for hydrogen-producing bacteria. Further, unlike pretreatment methods which inactivate hydrogen-consuming bacteria (e.g. methanogens) by chemical or physical means, thus prohibiting a switch from hydrogen to methane production, the use of a low moisture concentration (or severe dried condition) within the reactor vessel provides a system for selective switching between hydrogen and methane gas production.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a system for performing the method for selective production of hydrogen and methane from a biomass feedstock maintained under anaerobic conditions comprising a reactor vessel maintained under anaerobic conditions configured to receive biomass feedstock and a bacterial inoculum; a moisture monitor/controller configured to monitor and/or control a moisture level of contents of the reactor vessel; a gas collector configured to collect hydrogen and/or methane produced in the reactor vessel from digestion of the biomass feedstock; optionally, a pH monitor/controller configured to monitor and/or control pH of the contents of the reactor vessel; and optionally, a biomass pretreater configured to pretreat the biomass feedstock prior to introduction into the reactor vessel.