Accuracy of 22 DFT methods in predicting the energetics of test reactions involving MoO2(g), MoO3(g), MoO2(OH)(2(g)) and MoO2Cl2(g) is studied. Experimental heats of formation and CCSD(T) energies are used as the reference data. The most precise methods appear TPSS, PW91, PBE, M05, TPSSKCIS and BP86. Selected density functionals are then applied in investigations of isolated monooxo and dioxo Mo(VI) species on the silica surface. For the molybdena-silica models proposed in this work, the monooxo form is predicted to be more stable under dehydrated conditions than the dioxo species, irrespective of the DFT method used. (C) 2008 Elsevier B. V. All rights reserved.