EPR and ENDOR investigations have been carried out on polycrystalline samples of self(alpha)-irradiated PuO2C2O4. 3H(2)O (cumulative dose = 1 MGy). The Pu-239 (I = 1/2) hyperfine structure on the EPR signal of the CO2- radical ion has given clear evidence for participation of 5f electrons of Pu-239 in chemical bonding. The ENDOR spectra were used to identify unambiguousIy the hyperfine interaction between the unpaired electron on the CO, radical ion and the nuclear spin of Pu-239. In addition to the CO2- radical ion, EPR evidence was also obtained for the formation of O-2(-) during the autoradiolysis of PuO(2)C(2)O4 . 3H(2)O.