It is demonstrated that the gas-phase oxo-exchange of PaO2+ with water is substantially faster than that of UO2+, indicating that the Pa-O bonds are more susceptible to activation and formation of the bis-hydroxide intermediate, PaO(OH)(2)(+). To elucidate the nature of the water adduct of PaO2+, hydration of PaO2+ and UO2+, as well as collision induced dissociation (CID) and ligand-exchange of the water adducts of PaO2+ and UO2+, was studied. The results indicate that, in contrast to UO2(H2O)(+), the protactinium oxo bis-hydroxide isomer, PaO(OH)(2)(+), is produced as a gas-phase species close in energy to the hydrate isomer, PaO2(H2O)(+). CID behavior similar to that of Th(OH)(3) supports the assignment as PaO(OH)(2)(+). The gas-phase results are consistent with the spontaneous hydrolysis of PaO2 in aqueous solution, this in contrast to later AnO(2)(+) (An = U, Np, Pu), which forms stable hydrates in both solution and gas phase. In view of the known propensity for Th(IV) to hydrolyze, and previous gas-phase studies of other AnO(2)(+), it is concluded that the stabilities of oxo-hydroxides relative to oxide hydrates decreases in the order: Th(IV) > Pa(V) > U(V) > Np(V) > Pu(V). This trend suggests increasing covalency and decreasing ionicity of An-O bonds upon proceeding across the actinide series.