Gas-phase reactions of thorium hydroxide cations with water were studied in an ion trap and by density functional theory. The Th(OH)(2)(2+) ion adds five inner-shell water molecules. Addition of outer-shell water molecules to produce the Th(OH)(2)(2+)center dot(H2O)(6-8) yields Th(OH)(3)(+)center dot(H2O)(0-3) by intracluster proton transfer and elimination of a protonated water cluster, (H3O)(+)(H2O)(2). Facile hydrolysis of Th(IV) in these small hydrate clusters correlates with solution hydrolysis of Th(OH)(2)(2+)(aq) to Th(OH)(3)(+)(aq). The Th(OH)(3)(+) ion adds up to three inner-shell water molecules. For the other studied Th(IV) singly charged ions, ThO(OH)(+) exothermically hydrolyzes directly to Th(OH)(3)(+) by addition of a water molecule, ThO(O-2)(+) hydrolyzes to Th(OH)(3)(+) via nonthermalized Th(OH)(2)(O-2)(+), and Th(OH)(2)(O-2)(+) hydrolyzes to Th(OH)(3)(+)center dot(H2O) by a sequence that requires exothermic hydration prior to hydrolysis. Computed structures and energetics are in accord with the experimental observations.