The extraction of ²⁴¹Am and ¹⁵²Eu from alkaline carbonate solutions with solutions of functionalized thiacalix[4]arenes in m-nitrobenzotrifluoride was studied. The dependence of the radionuclide distribution ratios and separation factors on pH of the aqueous phase in the interval from 10 to 13.5, kind of the organic diluent, and position and electronic characteristics of functional groups in the thiacalixarene platform was examined. The composition of the extractable solvates of Am and Eu with functionalized thiacalix[4]arenes was determined by slope analysis. The hydrolytic stability of tert-butylthiacalix[4]arene in alkaline and acid solutions was evaluated. Thiacalix[4]arenes efficiently extract Am from alkaline solutions. The bromine-substituted thiacalix[4]arene exhibits the maximal selectivity, with the Am/Eu separation factor in extraction with this agent exceeding 30.