A process for separating a radionuclide such as radioactive Cu-64 from a mixture containing the radionuclide and a precursor element from which the radionuclide is formed. A weak acid solution is prepared which contains the mixture of the radionuclide and the element. The solution is contacted with a ligand which preferentially forms a complex with the radionuclide relative to the precursor element. The radionuclide complexed to the ligand is separated from the precursor element and released from the ligand by contacting the ligand with an acid.