Some renal stones were investigated by electron paramagnetic resonance of their untreated, UV-photolyzed and gamma-irradiated states. Powder X-ray diffraction technique indicated that the renal stones were made mainly from CaC2O4, MgC2O4, MgCO3 and NH4MgPO4 - 6H(2)O. Before radiation treatment, the renal stones yielded a signal that could be attributed to a C2O4- radical. UV-photolysis seems to slightly increase the intensity of this signal, but does not produce any new centres. Gamma-irradiation initially gives -CH2C(CH3)-R and CO2- radicals, and while the intensity of the -CH2C(CH3) R signal decreases, the intensity of the CO2- signal increases as time elapses. (c) 2004 Elsevier Ltd. All rights reserved.