The phenomenon of the incorporation of hydrogen into a single crystal of magnesium-doped alpha-alumina grown by the Verneuil method was studied using IR absorption and conductivity measurements. In the as-grown sample, a large wide band of IR absorption was observed in the range 2500-3500 cm(-1). This band was attributed to the stretching vibration of the OH bond formed by the incorporated hydrogen and host oxide ion. The reversible dissolution of hydrogen was confirmed by the equilibrium experiment in the hydrogen-containing atmosphere. The equilibrium amount of hydrogen evaluated by the integral absorbance for the specimen annealed in x% H2O-1% H-2-Ar was proportional to the half power of the partial pressure of water. The chemical diffusion of hydrogen was determined from the relaxation time of the dissolution phenomenon. The transport number of the proton roughly estimated from the H/D isotope effect observed in the electric conductivity showed that the proton was the major charge carrier in the specimen equilibrated with the gas mixture 20% H2O-1% H-2-Ar at 1373 K. The anisotropy observed in the IR absorption was found to be due to the fact that hydrogen enters between the neighboring O-O pair with a specific spacing and makes a hydrogen bond with them. Based on the experiment using polarized light, it was concluded that, among four kinds of O-O pairs with the different oxide ion octahedron spacings, two having a large angle to the basal plane were preferred for the site of hydrogen. The anisotropy observed in the chemical diffusion coefficient of hydrogen and in the electrical conductivity were reasonably explained based on the above configuration of the incorporated hydrogen. (C) 2003 Elsevier B.V. All rights reserved.