In this study, gamma-irradiated natural diatomite minerals were investigated by electron paramagnetic resonance (EPR) spectroscopy at ambient temperature. The paramagnetic radicals produced by gamma-irradiation in natural diatomite minerals at ambient temperature were free radicals. These centers were found to be perfectly attributed to CH3, PO42-, CO2-, organic (probably, humic or fluvic acid), SO2- and SO3- observable at below 20 mW microwave power. The g values of all the paramagnetic centers were obtained, and some of their a values were reported. The results were found to be consistent with the existing literature data and theoretical predictions for similar paramagnetic centers. (C) 2004 Elsevier B.V. All rights reserved.