Polybenzimidazole (PBI) membranes doped with excess of phosphoric acid (PA) were studied by H-1 and P-31 liquid probe NMR methods, under carefully controlled humidity. The results indicate that mobile (or "free") acid reside between the polymers chains and its properties are reflected from the H-1 NMR spectrum in which it appears as a broad single peak. The signal is strongly affected by the water adsorbed under different water activity. Upon drying, the membrane produces pyro-phosphoric acid (H4P2O7), which also appears as a broad characteristic peak in P-31 NMR spectrum at delta approximate to-12 pmm (vs reference of 85 wt % H3PO4). The similarities between the free acid and liquid solutions of PA enabled us to calculate the actual concentration of the mobile PA in the polymer using two methods. The first compares the observed chemical shift of protons in PBI/H3PO4 membranes to those of PA liquid solutions. In the second method, the concentration of the acid was evaluated from the ratio of ortho to pyro-phosphoric acid signals in the P-31-NMR spectrum. The validity and limitations of each method are discussed, as well as insight into PA species distribution in the mobile phase and in polymer bonded stats.