Understanding simple oxygen-exchange reactions is important to a variety of communities concerned with the chemistry of oxides with water. Limitations in the methods available for studying reactions at these oxidea??water interfaces, as well as difficulties in characterizing their structures, have led to the use of polyoxometalates (POMs) as model molecules. POMs are metal oxide ions composed of Group 5 and 6 metals. These ions constitute discrete and often soluble clusters than can be spectroscopically probed with great confidence. In addition, POMs are interesting in their own right, owing to their structural and chemical diversity, and are finding an increasing number of applications.[1a??5] We have been investigating the oxygen-isotope-exchange kinetics in these ions and aqueous solution by 17O-nuclear magnetic resonance (NMR) spectroscopy to help better recognize what controls the moleculea??water interface processes on the level of individual oxygen sites.[6a??10] These structures were chosen because the isotope-exchange reactions could be followed separately from dissociation or condensation of the structure.