The adsorption of a water molecule on a partially reduced TiO2 anatase (101) surface has been studied by first-principles molecular-dynamics simulations. At variance with the stoichiometric surface, dissociation of water close to the oxygen vacancy is energetically favored compared to molecular adsorption. However, no spontaneous dissociation was observed in a simulation of several picoseconds, indicating the presence of an energy barrier between the molecular and dissociated states. The free energy profile along a possible dissociation path has been determined through constrained molecular dynamics runs, from which a free energy barrier for dissociation of similar to0.1 eV is estimated. On the basis of these results, a mechanism for the dissociation of water at low coverage is proposed. (C) 2003 American Institute of Physics.