The oxidation of propane over alumina-supported V-Mn-K oxide catalysts has been investigated. The addition of K to vanadia-alumina decreases activity but increases significantly the selectivity to propene. This was found to be due to the poisoning of the acid sites of the alumina support, that convert propene in a number of organic byproducts. Conversely, the addition of Mn decreases both activity and selectivity. In both cases the coordination state of vanadyl sites is perturbed with an increase of the crystal field. FT-IR mechanistic studies support the idea that isopropoxides are intermediates in the formation of propene by propane oxydehydrogenation.