Polybenzimidazole (PBI) membranes doped with phosphoric acid are the most commonly used materials for high-temperature polymer electrolyte fuel cells. Despite this fact, the thermodynamics of the adsorption process over the completely accessible doping range is still a hot topic and only little is known on the chemical equilibria of the species inside the membrane as a function of the composition of the ternary system PBI - H₃PO₄ - H₂O. This includes details on the condensation equilibria, leading to the formation of diphosphoric acid and higher homologues species and on the proton transfer processes, i.e. on the dominant conduction mechanisms. As a first step to understand the thermodynamic equilibria in the ternary system PBI - H₃PO₄ - H₂O the phase diagrams for two different polymers, uncross-linked poly(2,2’-(m-phenylene)-5,5’-bibenzimidazole) (m-PBI) and a cross-linked commercial PBI derivate (Fumapem AM-55), were evaluated from own and from experimental data found in literature. In this study, miscibility gaps are identified and equilibrium lines separating one- and two-phase fields in the diagrams are estimated. In conclusion, we present new evidences on the interactions between H3PO4 and polymer chains, the polycondensation equilibria of H3PO4 and the implications on proton conductivity. References: [1] Uptake of protic electrolytes by polybenzimidazole-type polymers – adsorption isotherm and electrolyte/polymer interactions C. Korte, F. Conti, J. Wackerl, P. Damms, A. Majerus, W. Lehnert J. Appl. Electrochem., 2015, 45, 857-871 [2] A vibrational spectroscopic and DFT study of poly(2,5-benzimidazole) (ABPBI) –phosphoric acid interactions in HT-PEFC membranes G. A. Giffin, F. Conti, S. Lavina, A. Majerus, G. Pace, C. Korte, W. Lehnert, V. Di Noto Int. J. Hydrogen Energy, 2014, 39, 2776-2784 [3] Carbon NMR investigation of the polybenzimidazole-dimethylacetamide interactions in membranes for fuel cells. F. Conti, S. Willbold, S. Mammi, C. Korte, W. Lehnert, D. Stolten, New J. Chem., 2013, 37, 152-156. [4] Thermogravimetric and Spectroscopic Investigation of the Interaction between Polybenzimidazole and Phosphoric Acid A. Majerus, F. Conti, C. Korte, W. Lehnert, D. Stolten ECS Trans., 2012, 50(2), 1155-1165. [5] Raman study of the interactions polybenzimidazole – phosphoric acid in membranes for fuel cells F. Conti, A. Majerus, V. Di Noto, C. Korte, W. Lehnert, D. Stolten Phys. Chem. Chem. Phys., 2012, 14, 10022-10026.