1. Based on analysis of the IR spectrum of the hydrate-hydroxyl covering of alumina, OH groups of Al2O3 and coordinately bound water molecules have been identified which are sources of Bronsted acidity on the Al2O3 surface, which are able to form NH4+ ions when ammonia is adsorbed and to participate in ion exchange with Na+ ions. 2. The OH groups of Al2O3 giving rise to an absorption band above 3700 cm−1 are OH}-ions and are able to participate in exchange with chloride ions. 3. The disappearance from the spectrum of the high-frequency bands due to OH group vibrations when ammonia is adsorbed is due to the formation of hydrogen bonds between these OH groups and ammonia which is strongly adsorbed on the electron-acceptor centers of the surface.