Sputtered films of titanium dioxide several thousand angstroms thick and sandwiched between a layer of titanium and a layer of platinum are highly selective detectors of hydrogen. The resistance of the device decreases on exposure to hydrogen. At concentrations above 0.5% in air, the response is rapid and limited by lead resistance. At lower concentrations the response is proportional to concentration but relatively slow at room temperature. Moderate increases in temperature quicken the response. The detectors may be used as effective alarm devices at room temperature, with a‐c or d‐c circuitry that requires negligible power in the absence of hydrogen.