We report novel p-type behaviour of undoped SnO2 thin films irradiated with a 75 MeV Ni+ ion beam. Gas response of the irradiated films to NH3 (reducing) and NO2 (oxidizing) gases shows an increase and decrease in resistance, respectively, indicating p-type conduction that also increases with an increase in ion fluence. Photoluminescence spectroscopy of the irradiated films shows a strong yellow peak corresponding to interstitial oxygen ions. The observed p-type conductivity is attributed to the holes generated by these interstitial oxygen ions. The presence of interstitial oxygen ions is also confirmed by X-ray photoelectron spectroscopy. The conduction activation energy decreases with increasing the ion fluence, indicating an increase in hole density that supports the gas sensing and photoluminescence results. © 2008 Elsevier B.V. All rights reserved.