Cluster model studies by means of ab initio DFT method are performed to examine electronic properties of different surface O atoms in several V-O-X systems and to correlate them with catalytic behavior. Since chosen compounds vary in metal oxidation state and in X element, reactivity of differently coordinated surface O is studied as a function of different parameters such as coordination number, geometrical and chemical environment. In addition, the mobility of different surface oxygen species, formation of surface oxygen vacancies and the adsorption of O-2 molecule at the different vacancy is discussed on the examples of the V2O5 and MoO3 systems. (C) 2004 Elsevier B.V. All rights reserved.