The mechanism of hydrogen (H(2)) gas sensing in the range of 200-1000 ppm of RF-sputtered ZnO films was studied. The I-V characteristics as a function of operating temperature proved the ohmic behaviour of the contacts to the sensor. The complex impedance spectrum (IS) of the ZnO films showed a single semicircle with shrinkage in the diameter as the temperature increased. The best fitting of these data proved that the device structure can be modelled as a single resistance-capacitance equivalent circuit. It was suggested that the conductivity mechanism in the ZnO sensor is controlled by surface reaction. The impedance spectrum also exhibited a decreased in semicircle radius as the hydrogen concentration was increased in the range from 200 ppm to 1000 ppm. (C) 2010 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.