Swift heavy ion irradiation has been successfully used to modify the structural, optical, and gas sensing properties of SnO 2 thin films. The SnO 2 thin films prepared by sol–gel process were irradiated with 75MeV Ni + beam at fluences ranging from 1×10 11 ion/cm 2 to 3×10 13 ion/cm 2 . Structural characterization with glancing angle X-ray diffraction shows an enhancement of crystallinity and systematic change of stress in the SnO 2 lattice up to a threshold value of 1×10 13 ions/cm 2 , but decrease in crystallinity at highest fluence of 3×10 13 ions/cm 2 . 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 3.75eV to 3.1eV, and a broad yellow luminescence, respectively, with increase in ion fluence. Gas response of the irradiated SnO 2 films shows increase of resistance on exposure to ammonia (NH 3 ), indicating p-type conductivity resulting from ion irradiation.