We report a study of thermoreversible gelation of polybenzimidazole (PBI) in phosphoric acid (PA). ne PBI gel in PA exhibits fibrillar network morphology and reversible first order phase transition. The gelation rate is measured by the tube tilting method and found to depend both upon gelation concentration and gelation temperature. The UV-vis study demonstrates that the gelation process is a two-step process: conformational transformation and aggregation which produces crystallites for gel formation. The WAXS study supports the presence of crystallites in the gel. The PA doping level of the membrane increases significantly because of gelation.