Besides the three previously reported (3) surface species (i. e. outer, inner complex and a py+-species) which are formed on adsorption and desorption of pyridine on dehydroxylated alumina at temperatures above 200 °C, a fourth low temperature species is identified. The dependence of the surface concentrations of the various species and of the acid sites which they occupy is discussed usingPeris, surface model ofγ-Al2O3. It can be shown to be highly probable that the inner complex and the py+-species are formed solely on triplet oxide defects. These acid sites have to be distinguished with respect to there nearest neighbours. At least two types of triplet defects must be postulated; type A preferentially contains adjoined oxide ions and may possibly correspond toPeris α-sites, type B preferentially contains adjoined hydroxyl ions and therefore exhibits the higherLewis acidity. Whereas the py+species is formed on this latter type of triplet defect, outer and inner complexes presumably are formed on all types of triplet oxide vacancies.