The extraction properties of a new extractant, tetrahexyldicarbollylcobaltate, were studied. The reagent is stable in contact with alkaline solutions and permits (with the addition of PEG 400 or Slovafol 909) the extraction of both Sr-95 and Cs-137 from 1 M NaOH + 4 M NaNO3. The system studied was mainly 0.06 M reagent in isopropylbenzene + 1 vol% PEG 400. The extraction results compare well with the analogous reference system of nonsubstituted dicarbollylcobaltate in nitrobenzene. Possible ways of backextraction are discussed, and the system seems to be a candidate for further study for the separation of elements from high level waste in alkaline form and with a high NaNO3 content. The reagent and its application for the treatment of alkaline nuclear wastes was first proposed at Los Alamos National Laboratory, USA.