Thermoreversible gelation of polybenzimidazole (PBI) in phosphoric acid (PA) is investigated by studying the gel morphology, thermodynamics of the gelation, and gelation kinetics utilizing test tube tilting and UV-Vis spectroscopy techniques. Gelation kinetics studies reveal that both the gelation rate and critical gelation concentration (C(t=infinity)*) are function of gelation temperature (T(gel)) and the molecular weight of PBI. Highly dense fibrillar network morphology with large number of longer and thinner fibrils is obtained for higher gel concentration and higher molecular weight PBI. Both the gel melting (T(gm)) and gelation (T(gel)) temperature depend upon the gelation concentration and molecular weight of PBI. The presence of self-assembled chains of PA molecules, which help to produce the PBI crystallites, is observed from the thermodynamical study. I.R. and Raman studies prove the presence of strong hydrogen bonding interaction between the PBI and the PA molecules, and the free PA molecules in the gel network. The gelation occurs in two-step processes which include a slow rate determining conformational transition from coil to rod and followed by aggregation of rod via crystallization. The PA loading of PBI membrane obtained from the PBI-PA gel is significantly high compared to the conventional imbibing process membrane. The PBI gel membrane displays very high thermal and mechanical stabilities. The high acid loading and superb thermo-mechanical stability are due to the gel network structure of the membrane. The proton conductivity of the membrane at 160 degrees C and 0% relative humidity (RH) is similar to 0.1 S cm(-1), which is higher than the reported values in the literature for the PBI. The activation energy of the proton conduction is 14-15 kJ/mol indicating faster proton transfer by hopping process inside the gel network. (C) 2011 Elsevier Ltd. All rights reserved.