Standard Gibbs free energies for transfer (Delta G(tr)degrees) of actinyl ions (AnO(2)(z+).; z = 2 or 1; An: U, Np, or Pu) between an aqueous solution and an organic solution were determined based on distribution method combined with voltammetry for ion transfer at the interface of two immiscible electrolyte solutions. The organic solutions examined were nitrobenzene, 1,2-dichloroethane, benzonitrile, acetophenone, and 2-nitrophenyl octyl ether. Irrespective of the type of organic solutions, Delta G(tr)degrees of UO22+, NpO22+, and PuO22+ were nearly equal to each other and slightly larger than that of Mg2+. The Delta G(tr)degrees of Nu pO(2)(+) was extraordinary large compared with those of ordinary monovalent cations. The dependence of Delta G(tr)degrees of AnO(2)(z+) on the type of organic solutions was similar to that of H+ or Mg2+. The Delta G(tr)degrees of An(3+) and An(4+) were also discussed briefly. (C) 2011 Elsevier Ltd. All rights reserved.