Gamma-ray induced decomposition of the nitrate ion in solid solution of KBr crystal has been studied by infrared spectroscopy. The disappearance of the nitrate follows the rate law derived from the mechanism (1) NO3−→NO2− + O, (2) NO2− + O → NO3−, and (3) NO3− + 0 → NO2− + O2. At room temperatureki=l.3 ± 0.2 hr−1andk2/k3=70 ± l at a dose of 5.9 × 1019 eV hr−1g−1. The appearance of the nitrite ion follows the same rate law only at the initial stage of the reaction when its secondary destruction by the radiation is negligible. The nitrite ion itself decays by an apparent second order process but only about a third or less of its decomposition produces the nitrate ion.