Electrical response of anatase TiO2, films is discussed for different concentrations of CO gas (20-100 ppm) in a nitrogen gas ambient. We investigated temperature (100-300 degrees C and film thickness (100-1000 nm) effects for films deposited on glass, sapphire (0 0 0 1) and, Si(1 0 0) substrates. In general, there is a drop in resistance of the device when exposed to CO gas. Films deposited on sapphire showed a larger decrease in the resistance at 300 degrees C compared to those deposited on glass and silicon substrates. However, films grown on glass and silicon substrates showed a larger decrease in the resistance values for temperature around 200 degrees C when CO (ppm) values are greater than 40. The change in resistance of the films varies as square root of the CO gas concentration at 200 degrees C for films deposited on all the three substrates. In general, the decrease in resistance for thicker films is large compared to thinner ones, indicating participation of the bulk in the detection of CO gas. In the presence of O-2, film loses its sensitivity to CO gas when the concentration level of O-2 approaches 1800 ppm. (c) 2007 Elsevier B.V. All rights reserved.