A hydrogen safety sensor is presented, which provides high sensitivity and fast response when operated in air. The target application for the sensor is external deployment near systems using or producing high concentrations of hydrogen. The sensor is composed of a catalytically active metal-oxide sensing electrode and a noble metal reference electrode attached to an yttria-stabilized zirconia (YSZ) electrolyte. The sensing approach is based on the difference in oxidation rate of hydrogen on the different electrode materials. Results will be presented for a sensor using a sensing electrode of tin-doped indium oxide (ITO). Response to H-2 and cross-sensitivity to hydrocarbon and H2O are discussed. (C) 2004 Elsevier B.V. All rights reserved.