The effect of chemical composition on nickel-molybdenum catalysts in the oxidative dehydrogenation of n-hexane was studied. The catalysts consisted of the following phases: nickel oxide, molybdenum trioxide, nickel molybdate (alpha-NiMoO4 and beta-NiMoO4) and a solid solution of nickel in a lattice of nickel molybdate. Characterization of the catalysts was carried out using ICP-OES, BET, XRD, FTIR, SEM, TPR and XPS techniques. The effect of phase composition of the catalysts on their catalytic activity and selectivity to the products obtained in the temperature range 300-500 degrees C was also studied. The most selective catalyst for the synthesis of hexenes was the pure beta-NiMoO4, which gave 25% selectivity to 1-hexene and 10% selectivity to 2-hexenes and 3-hexenes at 9% conversion. The pure phases NiO and MoO3, although active in hexane ODH, showed poor selectivity to hexenes, particularly to 1-hexene at comparable conversions suggesting that they are not responsible for the catalytic activity of the NiO-MoO3 system. The pure P-phase was more selective towards the hexenes than the alpha-phase with the difference in the molybdenum coordination believed to be the reason for the different activities and selectivities of the two modifications of NiMoO4. The alpha-phase with MoO3 was more active, suggesting that a synergetic effect plays an important role in modifying catalytic activity. (C) 2009 Elsevier B.V. All rights reserved.