An EPR investigation ofγ-irradiated PETN (Pentaerythritol Tetranitrate) was conducted. Low temperature irradiation accumulates at least two different radicals. The more stable radical is the NO2 radical presumably produced by photon cleavage of the NO2 groups and/or by radical reactions in excited states resulting in detached NO2. The spectrum of the more reactive radical is obtained by computer substraction. Decay kinetics are proposed for the NO2 and the unknown highly reactive radical.