The orthophosphate anion, (HO)(2)PO2-, was produced in the gas phase by electrospray. The hydration equilibria (HO)(2)PO2-(H2O)(n-1) + H2O = (HO)(2)PO2-(H2O)(n) were determined after transfer of the ions into a reaction chamber containing known partial pressures of H2O, where thermal conditions prevailed. The Delta G degrees(n-1,n), Delta H degrees(n-1,n), and Delta S degrees(n-1,n) for steps (n-1,n) = (0,1) and (1,2) were obtained from equilibrium measurements in the 20-150 degrees C range. The ion concentration ratio was determined mass spectrometrically. The thermochemical data obtained are in good agreement with recent theoretical calculations. Under some conditions of the experiments, the equilibria (HO)(2)PO2-(H2O)(n) = PO3-(H2O)(n-1) + H2O should have been observed according to reports in the literature. However, the formation of the metaphosphate ion, PO3-, and its hydrates was not observed. In separate experiments with a different apparatus setup, the kinetic energy threshold for the collision-induced decomposition (HO)(2)PO2- = PO3- + H2O was determined. The activation energy barrier obtained was high. The significance of these results to the possible conversion of the orthophosphate to metaphosphate, when a few hydrating molecules are available, is discussed.