Six different exchange-correlation functionals were assessed in a quantum chemical study of the geometry and energetics of silanol, its conjugated base and both conformers of disiloxane, using Dunning's augmented correlation consistent basis set. It was found that the B3LYP and the B3PW91 hybrid functionals perform the best, their performance was comparable with MP2/VQZ calculations. A deprotonation energy of -365.4 kcal/mol for silanol was obtained at the B3PWB1/AVQZ level. For disiloxane, a very small difference between both disiloxane conformers, 0.037 kcal/mol (extra stabilization for the doubly staggered conformation) and 149.3 degrees for the very sensible Si-O-Si angle was found, or 154.7 degrees temperature corrected (300 K). degrees 2001 Elsevier Science B.V. All rights reserved.