Three different methods have been used to study the hydroxyls of montmorillonite, both as such and pillared with Al13 polycations. The methods are (i) formation of H-bonds with benzene, (ii) selective H/D exchange, and (iii) adsorption of pyridine. Besides non-acidic species, two reactive hydroxyls have been identified, characterized by stretching modes at 3738 and 3660 cm-1. The former is ascribed to a species Al-O-Si-OH, resembling the hydroxyl on silica, and the latter to a species Si-O-Al-OH, similar to extraframework species in zeolites. Both arise from sealing the clay sheet and the Al pillar; their acidity comes from a synergic effect between the two constituents, as similar species in both silica and alumina are definitely less acidic. No bridged species Si(OH)Al is formed. Pyridine adsorption shows that (i) only the Si-O-Al-OH species may transfer protons, (ii) at least two distinct Lewis acid sites are present, and (iii) one of the related pyridine species reacts with a nearby hydroxyl to yield alpha-pyridone.