The infrared spectra of polyatomic ions in dilute solid solution reflect interactions of the vibrational modes with the lattice environment. The spectra are usually characterized by very narrow bandwidths, and by frequency shifts and changes in absorption intensities which depend upon the host lattice. We have examined the spectra of a number of ions in solid solution in alkali halides. From this data, conclusions have been drawn relative to the symmetries of the ions and the lattice sites which they occupy. The ultimate goal is the use of complex ions as ``probes'' to yield information on crystal lattices. Certain of the ions produced in fused alkali halides are characteristic of chemical equilibria at high temperatures, and have not previously been studied. We have identified several impurities found in the spectra of single crystals from commercial sources. The infrared absorption offers the possibility of determining the types and concentrations of trace impurities in single crystals.