The structure of hydroxylated alumina surface is analysed by IR spectroscopy. The variety of OH groups differing in number and coordination of surrounding metal atoms, for the completely hydrated surface, either formed by ideal low-index planes, or complicated by crystal edges and corners or cation vacancies, is restricted to six, but grows dramatically upon dehydroxylation. The results account for the complex structure observed previously in the IR spectra of surface OH groups after high-temperature treatment, and enable us to explain specific properties of transition aluminas.