In the structure of spinel aluminas (γ, η, δ etc.), treated at moderate temperatures, a certain part of cation vacancies are always hydrated. Protons trapped in the octahedral and tetrahedral vacancies give rise to wide bands at about 3500 and 3300 cm^-1, respectively. These bands are not affected by CO or pyridine adsorption or by D₂O treatment at 300 K. Evacuation at temperatures higher than 750 K results in their disappearance, but after heating the sample in water vapour the structural hydroxyls can be partly restored. Treatment in D₂Ogives rise to the corresponding OD bands at 2590 and 2480 cm^-1, respectively. Complete saturation of all the cation vacancies by protons in the spinel lattice gives an ideal "protospinel" structure with the formula HAl₅O₈ or Al₂O₃·0.2 H₂O. The role of protonated vacancies and proton migration in the formation of alumina structure and their influence on the properties of oxides is discussed.