Phosphoric acid solutions progressively neutralized by sodium hydroxide are characterized by means of the R0(H) (-log a(H+)) acidity function and the R0(H2PO4-) (-log a(H2PO4-)), R0(HPO42-) (-log a(HPO4)2-) and R0(PO43-) (-log a(PO43-)) functions which represent the aptitude of these solutions to liberate the protons and the H2PO4-, HPO42-, and PO43- species, respectively. Their solvation properties are characterized by means of the solvation-transfer activity coefficients of the solutes which are calculated from the measured emf of the corresponding redox systems. The behavior of the water activity during the neutralization of 5.5 mol/L phosphoric acid is studied.