Abstract: A hydrogen gas sensor and/or switch fabricated from arrays nanowires composed of metal or metal alloys that have stable metal hydride phases. The sensor and/or switch response times make it quite suitable for measuring the concentration of hydrogen in a flowing gas stream. The sensor and/or switch preferably operates by measuring the resistance of several metal nanowires arrayed in parallel in the presence of hydrogen gas. The nanowires preferably comprise gaps or break junctions that can function as a switch that closes in the presence of hydrogen gas.