Annealing of acceptor dominated &agr;‐Al2O3atTsat= 1100–1500 °C in atmospheres containing hydrogen leads to dissolution of hydrogen, changing the material fromp‐ ton‐type when the concentration of hydrogen donors is larger than that of the acceptor impurities present. If oxygen equilibrium is maintained, the concentration of ionized donors depends onpH2as well aspO2(orpH2O). In measurements of dc conductivity in atmospheres not containing hydrogen, a contribution to the conductivity by mobile protons rapidly disappears as a result of polarization. Analysis of the rate of polarization leads to values for the concentration and mobility of mobile protons. In measurements in atmospheres with well‐defined hydrogen and oxygen fugacities, protons, native ions, and electrons and holes contribute continuously to the conductivity. Transference numbers of the various species and the corresponding partial conductivities were determined by emf measurements. The amount of dissolved hydrogen increases with decreasing temperature of saturation, increasing hydrogen pressure, and increasing oxygen pressure. Attempts to find detailed defect models explaining the results were only partially successful.