Polybenzimidazole (PBI) membranes doped with phosphoric acid are used as proton-conducting membrane in high-temperature polymer electrolyte fuel cells. There is no general model for the thermodynamics of the absorption process over the whole accessible doping range for all published data. The interactions between and polymer chains, the polycondensation equilibria of and the implications on proton conductivity are still largely unknown. In this study, we demonstrate that the uptake of a protic electrolyte by a polymer with basic moieties, i.e. a PBI-type polymer, can be described satisfactorily with a BET-like absorption isotherm. The absorption equilibria of the uptake process are analysed using literature data on the , and uptake of non-crosslinked m-PBI and AB-PBI as well as using our own investigations on the uptake of a commercial crosslinked PBI derivative, Fumapem AM-55. In addition to the thermodynamic data of the absorption process, Raman data on m-PBI taken from the literature and our own Raman investigations on Fumapem AM-55 are taken into account. It is possible to correlate domains in the absorption isotherms with specific features in the Raman spectra. Two stages for the uptake of a protic electrolyte can be distinguished: (i) the protonation of the polymer chains and thus coulombic interactions with the electrolyte anions, (ii) the formation of H bonds directly with the chains and with electrolyte molecules that are still absorbed.