The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of An(VI)O(2)(ClO4)(2) (An = U, Np, Pu): (UO2+)-O-V, (NpO2+)-O-V, (PuO2+)-O-V, (UO2)-O-VI(OH)(+), and (PuO2)-O-VI(OH)(+); abundances of the actinyl ions reflect the relative stabilities of the An(VI) and An(V) oxidation states. Gas-phase reactions with water in an ion trap revealed that water addition terminates at AnO(2)(+)center dot(H2O)(4) (An = U, Np, Pu) and AnO(2)(OH)(+)center dot(H2O)(3) (An = U, Pu), each with four equatorial ligands. These terminal hydrates evidently correspond to the maximum inner-sphere water coordination in the gas phase, as substantiated by density functional theory (DFT) computations of the hydrate structures and energetics. Measured hydration rates for the AnO(2)(OH)(+) were substantially faster than for the AnO(2)(+), reflecting additional vibrational degrees of freedom in the hydroxide ions for stabilization cif hot adducts. Dioxygen addition resulted in UO2+(O-2)(H2O) (n = 2, 3), whereas O-2 addition was not observed for NpO2+ or PuO2+ hydrates. DFT suggests that two-electron three-centered bonds form between UO2+ and O-2, but not between NpO2+ and O-2. As formation of the UO2+-O-2 bonds formally corresponds to the oxidation of U(V) to U(VI), the absence of this bonding, with NpO2+ can be considered a manifestation of the lower relative stability of Np(VI).