The kinetics of In2O3-based thin film gas sensor response to reducing and oxidizing gases was analyzed in this article. In2O3 films with thickness from 20 to 400 nm were deposited by spray pyrolysis. The influence of operating temperature, air humidity and film thickness on time constant of sensor response was reviewed. Model concepts that allow explanation of the specific character of the In2O3 interaction with reducing and oxidizing gases were proposed. It was concluded that water and oxygen adsorption/desorption processes are the main factors controlling the rate of response and recovery of the In2O3 gas sensors. Only during response to ozone in dry air the time of intergrain or bulk oxygen diffusion determines the sensor response time. It was established that reduction/reoxidation processes at the surface of In2O3 during gas detection are possible at temperatures as low as 120-150 degrees C. (c) 2006 Elsevier B.V. All rights reserved.