We report novel p-type behaviour in undoped SnO2 thin films irradiated with 75 MeV Ni+ ion beam. Gas response of the irradiated films with NH3 (reducing) and NO2 (oxidizing) gases shows an increase and decrease in resistance respectively indicating p-type conduction that also increases with increase in ion fluence. Photoluminescence spectroscopy of the irradiated films shows strong yellow peak corresponding to interstitial oxygen ions. The observed p-type conductivity is attributed to holes generated by these interstitial oxygen ions. Presence of interstitial oxygen ions is also supported by x-ray photoelectron spectroscopy. © 2008 IEEE.