The gas-phase proton affinity (PA) and the gas-phase basicity (GB) of orthophosphoric acid (H3PO4) have been determined by the kinetic method to be 833 and 800 kJ/mol, respectively. High level ab initio calculations at the G2(MP2) level of theory give a PA of 827 kJ/mol. A good agreement is observed between the G2(MP2) and density functional theory (using B3LYP) results. B3LYP/6-311 + G(2df,p) calculations and chemical ionization (CI) experiments have been performed in order to clarify the loss of water from protonated H3PO4. The ore-oxygen atom is confirmed as the most basic site, the hydroxy-oxygen protonated isomer O=P(OH)(2)(OH2)(+) being predicted to be 126 kJ/mol less stable than P(OH)(4)(+). The isomerization barrier connecting both isomers is calculated as 200 kJ/mol and the dissociation products, water and protonated metaphosphoric acid O=P(OH)(2)(+), are found to be 284 kJ/mol with regard to P(OH)(4)(+). The proton affinity-of metaphosphoric acid (HPO3) is also evaluated to be 712 kJ/mol at the G2(MP2) level of theory. In addition, a proton-bound complex [HO(O)PO ... H ... OH2](+) has been located as an intermediate for the elimination of HPO3 or H2O. The elimination of HPO3, which is not observed in CI experiments, was found to be more endothermic than the loss of water by 23 kJ/mol.