The demand for olefins increases in recent years. Oxidative dehydrogenation (ODH) of n-butane is possible alternative to classical dehydrogenation, steam cracking and fluid catalytic cracking processes. The role of particular VOX species supported on hexagonal mesoporous silica (HMS) in oxidative dehydrogenation (ODH) of n-butane was investigated on two sets of VOX-HMS catalysts prepared by wet impregnation and direct synthesis differing in amount and distribution of VOX species. The materials were characterized by XRF, N-2-BET isotherms, XRD, SEM, H-2-TPR, O-2-TPO and DR UV-vis spectroscopy and tested in ODH of n-butane in the range of temperature from 460 to 540 degrees C. The highest activity and selectivity to olefins were reached on materials with high content of isolated monomeric VOX units with tetrahedral coordination which are generated up to 4-5 wt.% of vanadium. The species with high degree of polymerization participate mainly on total oxidation reactions and those species are formed especially by wet impregnation. (C) 2011 Elsevier B.V. All rights reserved.