Swift heavy ion irradiation has been successfully used to modify the structural, optical, and gas sensing properties of SnO2 thin films. The SnO2 thin films prepared by sol-gel process were irradiated with 75 MeV Ni+ beam at fluences ranging from 1 × 1011 ion/cm2 to 3 × 1013 ion/cm2. Structural characterization with glancing angle X-ray diffraction shows an enhancement of crystallinity and systematic change of stress in the SnO2 lattice up to a threshold value of 1 × 1013 ions/cm2, but decrease in crystallinity at highest fluence of 3 × 1013 ions/cm2. Microstructure investigation of the irradiated films by transmission electron microscopy supports the XRD observations. Optical properties studied by absorption and PL spectroscopies reveal a red shift of the band gap from 3.75 eV to 3.1 eV, and a broad yellow luminescence, respectively, with increase in ion fluence. Gas response of the irradiated SnO2 films shows increase of resistance on exposure to ammonia (NH3), indicating p-type conductivity resulting from ion irradiation. © 2008 Elsevier B.V. All rights reserved.