Density functional theory calculations were performed to investigate the gas-phase reaction of Th atom with water. Three reaction pathways were identified, which leads to the formation of ThOH + H, ThO + H2, and H2ThO. The latter two are generated via the intermediate HThOH, and the H2ThO specie is generated from the isomerization of the HThOH intermediate. A thorough description of the reaction mechanism taking into account different possible spin states together with analysis of the electronic factors offer insights into the reactivity of the actinides atom. The obtained results are compared with the available experimental data. The three reaction pathways were found to be exothermic, in which the isomerization channel was observed with best thermochemical conditions around 123.9 kcal/mol. The nature of the chemical bonding evolution along the reaction pathways was studied using topological analysis including electron localization function, atoms-in-molecules, and natural bond orbital. © 2014 Wiley Periodicals, Inc.