The results of acidimetric titration, infrared spectroscopy and31P-NMR show that dialkylmonothiophosphinic acid, dialkylphosphinic acid and other phosphorus compounds form when Cyanex 301™ is damaged by Y-irradiation. H2SO4appears after a dose of l×l06Gy. The radiolytic stability of Cyanex 301 improves after purification. The results of extraction experiments demonstrate that the extraction selectivity of Cyanex 301 toward Am over the lanthanides decreases after irradiation. The purified Cyanex 301 can still separate tracer amount of Am from tracer amount of Eu till lx 105Gy while the commercial Cyanex 301 can do so only with l×l04Gy.