The surface structure of silicated aluminas, very good as catalysts for n-butene isomerization to isobutene, has been investigated by FT-IR spectroscopy of the silicate species, of the surface hydroxy-groups and of adsorbed pyridine. It has been concluded that silication results in the formation of a surface spinel phase where silicon substitutes for aluminum in tetrahedral sites. This phase exposes surface silanol groups that are not Bronsted acidic enough to protonate pyridine at room temperature. The increase in activity without loss in selectivity to isobutene is mainly attributed to the increased surface hydroxy-group concentration arising from silication, occurring without an increase in Bronsted acidity that would result in loss of selectivity and fast deactivation.