The low-frequency infrared and Raman spectra of sodium and potassium phosphate solutions have been measured and discussed. The geometries, energies, and vibrational frequencies of various isomers of PO43-(H2O)(n), n = 0-6 are calculated at various levels up to MP2/6-31+G*. These properties are studied as a function of increasing cluster size. The experimental and theoretical vibrational spectra are compared.