In the present article, the infrared spectrum of polybenzimidazole (PBI) in the dry and hydrate forms has been studied both experimentally and theoretically to improve the interpretation of its complex features, especially in the ν(NH)/ν(OH) range, which is extensively affected by sorbed water and temperature. Time-resolved Fourier-transform infrared spectroscopy measurements were performed to monitor H2O sorption, whereas the temperature behavior was investigated by in situ measurements in the 100-450 °C range. Density functional theory calculations on simplified models of dry and hydrated PBI showed good agreement with experimental results and allowed a reliable interpretation of the observed effects. The combined experimental/computational analysis provided relevant structural information which suggested the possibility of modifying the diffusion properties of PBI and motivated further experimental activities.