The adsorption mechanism of water on the hydroxylated (001) plane of alpha-Al2O3 was studied by measuring adsorption isotherms and GCMC simulations. The experimental adsorption isotherms for three oc-Al2O3 samples from different sources are typical type II, in which adsorption starts sharply at low pressures, suggesting a high affinity of water to the Al2O3 surface. Water molecules are adsorbed in two registered forms (bilayer structure). In the first form, water is registered at the center of three surface hydroxyl groups by directing a proton of the water. In the second form, a water molecule is adsorbed by bridging two of the first-layer water molecules through hydrogen bonding, by which a hexagonal ring network is constructed over the hydroxylated surface. The network domains are spread over the surface, and their size decreases as the temperature increases. The simulated adsorption isotherms present a characteristic two-dimensional (2D) phase diagram including a 2D critical point at 365 K, which is higher than that on the hydroxylated Cr2O3 surface (319 K). This fact substantiates the high affinity of water molecules to the oc-Al2O3 surfaces, which enhances the adsorbability originating from higher heat of adsorption. The higher affinity of water molecules to the alpha-Al2O3 (001) plane is ascribed to the high compatibility of the crystal plane to form a hexagonal ring network of (001) plane of ice Ih. (C) 2011 Elsevier Inc. All rights reserved.