A solid-state electrochemical sensor has been developed for hydrogen leak detection in ambient air. The sensor uses an yttriastabilized zirconia (YSZ) electrolyte with one tin-doped indium oxide (ITO) and one Pt electrode. Excellent sensitivity, and response time <= 1 s, are reported for hydrogen gas in the concentration range of 0.03 to 5.5% in air. Cross-sensitivity to water vapor and CO2 are shown to be low. The response to methane, a potentially significant source of interference for such a sensor, is significantly smaller than that for hydrogen. The sensor shows good reproducibility and was unaffected by thermal cycling over the course of this investigation (similar to 240 h of operation). The effects of ITO electrode thickness and thermal aging, and the sensing mechanism, are discussed. The sensor is intended for monitoring potential hydrogen leakage in vehicles powered by hydrogen fuel. (c) 2005 The Electrochemical Society. All rights reserved.