We investigated the reaction between a sodium atom and water molecules in both small and medium-size vibrationally excited water clusters using ab initio molecular dynamics simulations. Formation of NaOH was observed in small (n = 4, 5) clusters, while water dissociation and subsequent geminate recombination accompanied by a transient formation of a Na+-OH pair occurred in a 34 water cluster. Our results show that the initial step of the vibrationally excited reaction between a single sodium atom and water does not shut off in larger clusters and that it can also occur in the bulk water, however, more sodium atoms are likely required to stabilize the product. (C) 2008 Elsevier B. V. All rights reserved.