The hydrolysis reaction of the tetravalent thorium (Th4+) ion was investigated by ab initio theoretical calculations on hydration complex models. The transition state structure was optimized, and the intrinsic reaction coordinate was traced. The results showed that the hydrolysis is highly exothermic and the transition state is close to the reactant. Reaction dependency upon the number of non-reactive water molecules was examined. (C) 2003 Elsevier Science B.V. All rights reserved.