The study of heavy elements produced in underground thermonuclear explosions requires the separation of trace quantities of actinide elements from several hundred to several thousand grams of fused rock containing the products from about 1017 fissions. After the sample is pulverized and dissolved in HNO3, HClO4, and HF, fluoride insoluble salts are precipitated. These are redissolved, and the actinides and lanthanides are extracted into tributyl phosphate from a solution that is highly salted with Al(NO3)3. The actinides and lanthanides are back-extracted intc water and then extracted into di-2-ethylhexyl phosphoric acid. Recovery from di-2-ethylhexyl phosphoric acid is achieved by esterification with decanol. The actinides are separated from the lanthanides by elution from a cation-exchange resin column with an ethanol-hydrochloric acid solution. Individual actinides are separated by elution from a cation-exchange resin column with α-hydroxyisobutyric acid.