EPR and ENDOR investigations have been carried out on polycrystalline samples of self(α)-irradiated PuO 2 C 2 O 4 .3H 2 O (cumulative dose = 1 MGy). The 2 3 9 Pu (I=1/2) hyperfine structure on the EPR signal of the CO 2 - radical ion has given clear evidence for participation of 5f electrons of 2 3 9 Pu in chemical bonding. The ENDOR spectra were used to identify unambiguously the hyperfine interaction between the unpaired electron on the CO 2 - radical ion and the nuclear spin of 2 3 9 Pu. In addition to the CO 2 - radical ion, EPR evidence was also obtained for the formation of O 2 - during the autoradiolysis of PuO 2 C 2 O 4 .3H 2 O.