In order to evaluate the transmutation behavior of americium under fast neutron spectra, two irradiated 241 Am samples (No. 69, No. 70) were analyzed by radiochemical methods. About 100 mg of 241 Am (Am oxide, 99.9%) samples encapsulated in a small vanadium capsule were irradiated in the experimental fast reactor JOYO for 275 effective full power day (EFPD) by the fast neutron flux of 1:08 Â 10 15 (No. 69, in the reflector region) and 3:25 Â 10 15 (No. 70, at the center core) nÁcm À2 s À1 (E ! 0:1 MeV). After dissolution of these samples, americium, curium, and plutonium were chemically separated and the isotopic composition was determined by alpha and gamma-ray spectrometries and mass spectroscopy.