We demonstrate that phosphate solutions can be efficiently vitrified using this protocol and studied by low-temperature 31 P solid-state NMR. We show that the different phosphate species existing in aqueous solution (PO4 3-, HPO4 2-, H2PO4 -and H3PO4) can be distinguished depending on the pH. If the isotropic components of these species severely overlap at B0 = 7T and T = -120°C, it is possible to differentiate them based on their CSA parameters, which are characteristic of their protonation degree. The 31 P CP MAS experiments allow the determination of the proportion of each species as a function of pH and, consequently, enable to plot the speciation diagram in our conditions of analysis. We observe that the "apparent pKa," i.e., the pH values for each chemical equilibrium, is slightly influenced by the low temperature and possibly the preparation conditions.