To understand the mechanism determining the over long response time of porous alumina humidity sensors, the study of the interaction between water and the anodic alumina was begun by determining the adsorbed water concentration at room temperature and pressure of the most utilized alumina-the sulfuric porous anodic alumina. The adsorbed water concentration in a dissociated state can be estimated as 100 OH nm-2, a constant value in a large domain of preparation conditions : 8.4-27 wt.% sulfuric acid concentration, +10 to -15-degrees-C, and 40-400 A m-2 anodic current density.