In this paper, we report the gas sensing properties of sol-gel derived Fe-doped nano-crystalline SnO2 thin films. Films were doped with Fe in two different concentrations (2 and 5 wt.%). The crystallite size was found to decrease with increase in stacking fault density resulting from increasing Fe content in the SnO2 films. Gas sensing characteristics were investigated for different target gases such as carbon monoxide (CO), ammonia (NH3) and ethanol (C2H5OH). The films with 2% Fe content showed high response and excellent selectivity for CO compared to other gases at 200 °C. But, at 5% Fe content, the response factor to CO decreased while it increased for NH3 and ethanol. However, faster response and recovery times were observed for CO at higher Fe content in the films. These results have been correlated with the defect chemistry and crystallite size effect resulting from Fe incorporation in the SnO2 thin films. © 2006 Elsevier B.V. All rights reserved.