The intrinsic hydration of three monopositive uranyl-anion complexes (UO 2 A) ϩ (where A ϭ acetate, nitrate, or hydroxide) was investigated using ion-trap mass spectrometry (IT-MS). The relative rates for the formation of the monohydrates [(UO 2 A)(H 2 O)] ϩ , with respect to the anion, followed the trend: Acetate Ն nitrate ӷ hydroxide. This finding was rationalized in terms of the donation of electron density by the strongly basic OH Ϫ to the uranyl metal center, thereby reducing the Lewis acidity of U and its propensity to react with incoming nucleophiles, viz., H 2 O. An alternative explanation is that the more complex acetate and nitrate anions provide increased degrees of freedom that could accommodate excess energy from the hydration reaction. The monohydrates also reacted with water, forming dihydrates and then trihydrates. , for which recent theoretical studies [5][6][7][8] have suggested that strong interactions between the cation and solvent molecules, with significant charge transfer, cause the solvating species to behave like equatorial ligands. Therefore, specific interaction with solvent is likely to influence the physico-chemical behavior of the uranyl ion and its complexes in the environment. Unfortunately, explicit control over the interactions of solvent and non-solvent ligands with the uranyl ion is difficult, which makes the study of species-dependent uranium behavior complicated. To gain a better understanding of the intrinsic interactions between different uranium species and solvent, we have turned to investigations of uranyl-anion complexes in the gas phase. Recent studies have shown that ion-trap mass spectrometry can be used to probe intrinsic metal and metal-complex chemistry by exposing metal species to reagent gases deliberately added to the ion trap [9 -20], or to adventitious H 2 O present in the He bath gas used to collisionally cool ions and improve trapping efficiency [21][22][23][24]. Further motivating the present study is the fact that ion traps will shortly be used to characterize actinide speciation in samples from radioactive waste disposal sites, and a concise understanding of actinide behavior will be critical for correctly interpreting environmental data [25].We recently demonstrated that electrospray ionization (ESI) and multiple stage tandem mass spectrometry (MS) can be used to generate and characterize "solvated", gas-phase complexes featuring the uranyl ion coordinated by hydroxide, nitrate or a series of alkoxides [26]. In the pilot study reported here, ESI was used to produce gas-phase ions from solutions containing uranyl nitrate or uranyl acetate in deionized H 2 O. The dominant species generated by ESI were those having