Polybenzimidazoles (PBIs) have been investigated for use as gas separation membranes due to their thermal stability and good H-2/CO2 separation performance at elevated temperatures. However, PBIs are hydrophilic, and effects of water vapor on PBI separation performance are essentially unexplored. This study investigates water sorption, diffusion, and dilation of a commercial PBI, Celazole, and a recently synthesized sulfonyl-containing PBI, TADPS-TPA. Water binds strongly to both polymers and requires elevated temperatures for removal. Water vapor sorption in Celazole follows dual-mode behavior up to unit activity, while upward concavity is observed at high activities in the TADPS-TPA sorption isotherm. Both PBIs swell significantly in water, and desorption hysteresis suggests water-induced conditioning of the PBI matrix. Model fits of dilation behavior enable estimation of partial molar volume and water-accessible fractional free volume (FFV) as a function of water activity. Water vapor diffusion coefficients increase with increasing vapor activity, which correlates with water-accessible FFV.