This paper documents the influence of the water saturation level and the pore size distribution of porous materials on the diffusion of hydrogen. The materials are two monomodal materials: geopolymers and Vycor glasses. First, the pore size repartition is measured by means of mercury intrusion porosimetry and nitrogen adsorption porosimetry. Next, the materials are characterized through their water sorption isotherms. Finally, the diffusion coefficients measured as a function of the water saturation degree allows to quantify the impact of water on the gas diffusion through the materials. The results highlight the existence of a threshold value of saturation degree which leads to a decrease of two orders of magnitude in the hydrogen diffusion coefficient. (C) 2013 Elsevier Inc. All rights reserved.