Two pure anatase preparations and one sulphate-contaminated preparation have been treated in vacuo at different temperatures, focussing the attention on the earliest stages of dehydration as well as on rehydration. The hydrated layer is made up of hydroxyl groups and of un-dissociated water molecules coordinated at surface cationic centres. The vacuum removal of the latter component, which starts at low temperatures and is complete at some 470 K, creates strong Lewis-acidic sites that reversibly chemisorb CO at ambient temperature up to a coverage of 10–20%. This CO_ads species is very sensitive to inductive effects produced by surface species, including CO itself, and is quite useful in revealing the dehydration/rehydration stage of the surface as well as any alteration of the distribution between OH-bearing and H₂O-bearing centres, produced by the vacuum thermal treatments. The vacuum removal of OH groups, which starts at temperatures as high as 450 K, creates only a small number of Lewis acidic sites that are strong enough to chemisorb CO. Sulphate contamination, which derives from the preparative route of the anatase, severely modifies the surface properties, in that it eliminates virtually all of the surface OH centres and produces strong inductive effects through the solid that are monitored by the CO species adsorbable at the sites which, when hydrated, coordinate water with a higher adsorption heat and a higher CO stretching frequency.