Gas-phase reactions of pentavalent metal dioxide cations (MO2+)-O-V with water were studied experimentally for M = V, Nb, Ta, Pr, Pa, U, Pu, and Am. Addition of two H2O can occur by adsorption to yield hydrate (H2O)(2)(MO2+)-O-V or by hydrolysis to yield hydroxide M-V(OH)(4)(+). Displacement of H2O by acetone indicates hydrates for Pr-V, U-V, Pu-V, and Am-V, whereas nondisplacement indicates hydroxides for Nb-V, Ta-V, and Pa-V, . Computed potential energy profiles agree with the experimental results and furthermore indicate that acetone unexpectedly induces dehydrolysis and displaces two H2O from (H2O)VO(OH)(2)(+) to yield (acetone)(2)VO2+. Structures and energies for several M-V, as well as for Th-IV and U-VI, indicate that hydrolysis is governed by the involvement of valence f versus d orbitals in bonding: linear f-element dioxides are more resistant to hydrolysis than bent d-element dioxides. Accordingly, for early actinides, hydrolysis of Th-IV is characteristic of a 6d-block transition metal; hydration of U-V and U-VI is characteristic of 5f actinyls; and Pa-V is intermediate between 6d and 5f. The praseodymium oxide cation (PrO2+)-O-V is assigned as an actinyl-like lanthanyl with properties governed by 4f bonding.