The oxidative dehydrogenation (OXDH) of n-butane and 1-butene on undoped and K-doped alumina-supported vanadia catalysts has been studied. The low selectivity to OXDH products on alumina-supported vanadia catalysts is a consequence of the isomerization of olefins (low temperatures) and the formation of carbon oxides (high temperatures) on acid sites. The presence of potassium results in a decrease of the number of acid sites and a higher selectivity to OXDH products from both n-butane and 1-butene. Infrared spectroscopy data of 1-butene adsorbed on the catalysts suggest the presence of different adsorbed species: (i) O-containing species on the undoped catalyst, or (ii) adsorbed butadiene on K-doped catalyst. A reaction network including parallel and consecutive reactions is proposed.