The structures of U(IV) and Np(IV) hydrates in aqueous solution have been studied using the relativistic density functional theory calculations. By comparing the dissociation energies of the hydrates that have different coordination numbers, it was concluded that U4+ ion has 9 water molecules in the first coordination sphere. Np4+ ion has 8 or 9 water molecules coordinated in the first hydration sphere. (C) 2004 Elsevier B.V. All rights reserved.