Phosphate (PO<sub>4</sub><sup>3−</sup>) solutions in water and heavy water have been studied by Raman and infrared spectroscopy over a broad concentration range (0.0091–5.280 mol/L) including a hydrate melt at 23 °C. In the low wavenumber range, spectra in R-format have been constructed and the R normalization procedure has been briefly discussed. The vibrational modes of the tetrahedral PO<sub>4</sub><sup>3−</sup>(aq) (T<sub>d</sub> symmetry) have been assigned and compared to the calculated values derived from the density functional theory (DFT) method for the unhydrated PO<sub>4</sub><sup>3−</sup>(T<sub>d</sub>) and phosphate–water clusters: PO<sub>4</sub><sup>3−</sup>·H<sub>2</sub>O (C<sub>2v</sub>), PO<sub>4</sub><sup>3−</sup>·2H<sub>2</sub>O (D<sub>2d</sub>), PO<sub>4</sub><sup>3−</sup>·4H<sub>2</sub>O (D<sub>2d</sub>), PO<sub>4</sub><sup>3−</sup>·6H<sub>2</sub>O (T<sub>d</sub>), and PO<sub>4</sub><sup>3−</sup>·12H<sub>2</sub>O (T), a cluster with a complete first hydration sphere of water molecules. A cluster with a second hydration sphere of 12 water molecules and 6 in the first sphere, PO<sub>4</sub><sup>3−</sup>·18H<sub>2</sub>O (T), has also been calculated. Agreement between measured and calculated vibrational modes is best in the case of the PO<sub>4</sub><sup>3−</sup>·12H<sub>2</sub>O cluster and the PO<sub>4</sub><sup>3−</sup>·18H<sub>2</sub>O cluster but far less so in the case of the unhydrated PO<sub>4</sub><sup>3−</sup> or phosphate–water cluster with a lower number of water molecules than 12. The asymmetric, broad band shape of <i>v</i><sub>1</sub>(a<sub>1</sub>) PO<sub>4</sub><sup>3−</sup> in aqueous solutions has been measured as a function of concentration and the asymmetric and broad band shape was explained. However, the same mode in heavy water has only half the full width at half-height compared to the mode in normal water. The PO<sub>4</sub><sup>3−</sup> is strongly hydrated in aqueous solutions. This has been verified by Raman spectroscopy comparing <i>v</i><sub>2</sub>(H<sub>2</sub>O), the deformation mode of water, and the stretching modes, the <i>v</i><sub>1</sub>OH and <i>v</i><sub>3</sub>OH of water, in K<sub>3</sub>PO<sub>4</sub> solutions as a function of concentration and comparison with the same modes in pure water. A mode at ∼240 cm<sup>−1</sup> (isotropic R spectrum) has been detected and assigned to the restricted translational mode of the strong hydrogen bonds formed between phosphate and water, P–O…HOH. In very concentrated K<sub>3</sub>PO<sub>4</sub> solutions (<i>C</i><sub>0</sub> ≥ 3.70 mol/L) and in the hydrate melt, formation of contact ion pairs (CIPs) could be detected. The phosphate in the CIPs shows a symmetry lowering of the T<sub>d</sub> symmetry to C<sub>3v</sub>. In the less concentrated solutions, PO<sub>4</sub><sup>3−</sup>(aq) solvent separated ion pairs and doubly solvent separated ion pairs exist, while in very dilute solutions fully hydrated ions are present (<i>C</i><sub>0</sub> ≤ 0.005 mol/L). Quantitative Raman measurements have been carried out to follow the hydrolysis of PO<sub>4</sub><sup>3−</sup>(aq) over a very broad concentration range. From the hydrolysis data, the pK<sub>3</sub> value for H<sub>3</sub>PO<sub>4</sub> has been determined to be 12.45 at 23 °C.

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