This work describes the design and characterization of a micromachined thermal conductivity sensor for detection of hydrogen in automotive fuel cell systems. Thermal and gas-sensing properties are investigated via simulations and experiments. The manufactured sensors consist of a thin dielectric membrane as carrier structure for a platinum beater and temperature sensor. Membrane and heater size were varied to examine their influence on sensitivity, power consumption and thermal response. Based on a sensor element with a membrane size 1-5 mm x 1.5 mm a hydrogen sensor prototype with a detection limit of 0.2% hydrogen in air was achieved. (C) 2002 Elsevier Science B.V All rights reserved.