Effect of additives (A) comprising main group elements (K, P) and transition metals (Ni, Cr, Mo) to VOx/SiO2 catalysts on their catalytic behavior in oxidative dehydrogenation (ODH) of propane are studied. The experimental findings are compared with results of quantum chemical calculations performed for a model O = V-O-V* cluster in which V* atom is replaced by an atom of A. The selectivities to propene found in experiments change on introducing the additives, decreasing in the sequence VK(64) > VCr(38) > VMo = VNi(32) > VP(28) - VO(27). Quantum chemical calculations indicate the change in the electron density on the atoms of the active O = V-O groups for the clusters with the additives' atoms. The extent of the electron transfer from A towards the active group (the increase in the centre basicity) follows the sequence of the decreasing selectivity to propene, confirming the role of acid-base properties in controlling the selectivity in ODH of propane. Results of calculations show also an exceptional behavior of the potassium (the most effective additive for the increase in the selectivity in experiments); in contrast to all other additivies, K transfers electrons towards both V and 0 atoms in the active group, and does not form a definite O-K bond, but is rather "adsorbed" on the cluster. (C) 2003 Elsevier B.V. All rights reserved.