- Abstract EPR active species were found in irradiated C 60 thin films at room temperature both as-grown and after irradiation by light. The samples exhibit a singlet Lorentzian line with g = 2.0026 ± 0.0002 and Δ H pp = 0.124 ± 0.005 mT which may be attributed to holes localized at rapidly rotating C 60 molecules, i.e. C 60 + free radicals. The bulk density of the C 60 + radicals in the as-grown samples depends on the conditions of the sample preparation and reaches 1 spin per 10 000 molecules for fast-grown films. Annealing at 570 K leads to a rapid decrease in that density. Irradiation of the C 60 thin films by visible and UV light leads to a significant increase in the number of the same C 60 + radicals. The total number of photoinduced defects depends on the light intensity and time of exposure. The irradiation by visible light is active only in the presence of oxygen whereas the UV irradiation acts also when the sample is kept in an inert atmosphere. At ambient conditions the C 60 + radicals are metastable with life time of several months.