Activated and non-activated states of the zeolites NH4NaY and NH(4)CaA, and their NH; forms were prepared with an (NH4)(2)SO4 solution and were investigated by EPR after gamma-irradiation. The formed radicals were attributed to O-2(-) supposed to be trapped near a trigonally coordinated framework of Al-27(27)(I = 5/2). Anisotropic O-2(-) spectra were observed, consisting of hyperfine structure due to the Al framework, even at room temperature. This was attributed to the scarceness of NH4+ sites or trap sites because of the preparation of the NH4+ forms of the zeolites with an (NH4)(2)SO4 solution. The crystal field splittings were found and interpreted.