Ab initio calculations have been performed to study the structures of thorium(IV) hydrate and its hydrolysis products in aqueous solution. The conductor-like polarizable continuum model (CPCM) has been used to perform geometry optimization calculations in aqueous solution. The calculated results demonstrate that the molecule geometries obtained in solvent are generally consistent with the experiments. The coordination number of thorium(IV) hydrolysis products has been investigated. The effect of the relativistic effective core potential (RECP) on the structures is also discussed. (C) 2003 Elsevier Science B.V. All rights reserved.