The mechanism of the reactions of W and W + with the water molecule have been studied for several lower-lying electronic states of tungsten centers at the CCSD(T)/6-311G(d,p)+SDD and B3LYP/6-31G(d,p)+SDD levels of theory. It is shown that these reactions are essentially multistate processes, during which lower-lying electronic states of the systems cross several times. They start with the formation of initial prereaction M(H 2O) complexes with M-H 2O bonding energies of 9.6 and 48.2 kcal/mol for M = W and W +, followed by insertion of the metal center into an O-H bond with 20.0 and 53.3 kcal/mol barriers for neutral and cationic systems, respectively. The overall process of M + H 2O → t-HM(OH) is calculated to be highly exothermic, 48.4 and 48.8 kcal/mol for M -W and W +. From the HM(OH) intermediate the reaction may proceed via several different channels, among which the stepwise HM(OH) → HMO + H → (H) 2MO and concerted HM(OH) → (H) 2MO pathways are more favorable and can compete (energetically) with each other. For the neutral system (M = W), the concerted process is the most favorable, whereas for the charged system (M = W +), the stepwise pathway is slightly more favorable. From the energetically most favorable intermediate (H) 2MO the reactions proceed via H 2-molecule formation with a 53.1 kcal/mol activation barrier for the neutral system. For the cationic system, H-H formation and dissociation is an almost barrierless process. The overall reaction of W and W + with the water molecule leading to H 2 + MO formation is found to be exothermic by 48.2 and 39.8 kcal/mol, respectively. In the gas phase with the collision-less conditions the reactions W( 7S) + H 2O → H 2 + WO( 3∑ +), and W+( 6D) + H 2O → H 2 + WO +( 4∑ +) are expected to proceed via a 10.4 and 5.1 kcal/mol overall energy barrier corresponding to the first O-H dissociation at the TS1. On the basis of these PESs, we predict kinetic rate constants for the reactions of W and W + with H 2O.