Oxidation of cyclopentadiene over vanadium oxide and its mixed oxides has been studied and correlated to changes of vanadium valence. There are various organic products containing one or two oxygen atoms besides maleic anhydride. The catalyst V-Ag-O shows the highest selectivity to maleic anhydride. The rusults indicate that the reaction follows the redox mechanism. The additives are thought to moderate the tendency of an oxide to donate its oxygen.