We report the first synthesis of 225Ac (t 1/2 = 10 days) endohedral fullerenes,225Ac@C60. The 225Ac@C60 was produced with a 12 ± 2% efficiency by applying an electrical arc discharge between a source of α-particle emitter 225Ac (∼1 mCi, electroplated on a Pt disk) and a thin coat of "preformed" C60 on an Al disk (C60 thickness of ∼0.25 mg/cm2). After formation by electrical arc discharge, the resulting radiofullerenes on the Al disk were dissolved in toluene under anaerobic conditions and converted to a malonate derivative using the Bingel reaction. Subsequent to repeated washings of the organic phase with dilute acidic solutions to remove exohedral 225Ac, ∼45% of 225Ac activity was retained in the organic phase, which resisted extraction into the aqueous phase. Failure to extract the 225Ac from the organic phase provided definitive evidence that the 225Ac is located inside of the fullerene. The formation of 225Ac@C60 was further confirmed using a classical hot-atom chemistry technique in which the organic phase containing purified endohedral 225Ac@C60 malonate was contacted with fresh dilute acid to repeatedly extract the ionic 4.8 m 221Fr and 45.6 m 213Bi activities (decay daughters of 225Ac), which were released by molecular disruption due to nuclear recoil. The result from the extraction experiments was further supported by a series of thin-layer chromatography and high-pressure liquid chromatography analysis of the organic phase containing 225Ac@C60 or 225Ac@C60 malonate. Taken together, studies show that, like polydentate chelators, single-wall fullerenes are not capable of retaining the 225Ac decay daughters.