The oxidative dehydrogenation of ethane, propane and n-butane over a vanadia-molybdena-niobia catalyst has been compared with propane substituted in the 2 position with a methyl (isobutane) or a benzyl (isopropylbenzene) group. In the normal paraffins, selectivity to the olefin was found to decrease as the chain length increased. Substitution at the propane 2 position increased selectivity, particularly in the case of isopropylbenzene. The results are explained partially in terms of over oxidation of the product olefin and partially in terms of structural effects in the parent hydrocarbon influencing the olefin-producing oxidative dehydrogenation reaction.