Ab initio periodic DFT calculations in combination with experimental FTIR of adsorption pyridine investigations have been used to study the acid-base properties of vanadium doped zeolite materials. It is evidenced that VO-H groups of V(V) and V(IV) framework sites are more acidic than SiO-H groups present in siliceous zeolites. It is proposed for the first time that the protonation of the penta coordinated V site can lead to the formation of a stable vanadyl group containing site through proton exchange, which is expected to be the inter-conversion path between both sites. We report in this paper protonation and deprotonation energies of different vanadium sites, and a geometry of experimentally observed Lewis and Bronsted acid sites is proposed. (C) 2009 Elsevier B.V. All rights reserved.