Vibrational pre-dissociation spectroscopy combined with mass spectrometry has been used to obtain the infrared spectra of mass-selected cluster ions. The onset of new spectroscopic features, as a function of solvent number, has been shown to correspond to specific structural changes such as the filling of solvent shells and the formation of hydrogen bonds. These small finite systems offer a number of advantages over traditional solution-based measurements and perhaps are the most useful way to test current theoretical calculations and models. Whereas thermodynamic properties, derived from high-pressure mass spectrometry measurements, give some indication of solvent shell size, vibrational spectra suggest a more complex picture. When combined with recent infrared studies of size-selected neutral clusters, considerable detail of the solvent structure about the ion can be obtained. The solvation of the alkali-metal ions Na and Cs by a number of solvents, including water, will be discussed. Future directions and applications will be presented.