The partial oxidation of C-5 hydrocarbons was investigated over unpromoted and alkali-promoted V2O5 catalysts. Products of this mild oxidation reaction include both maleic and phthalic anhydride. The apparent effect of alkali promotion is to facilitate the loss of surface oxygen species which are important in the activation of alkanes and the complete oxidation of intermediate species. Studies performed using l-pentene as feed have indicated that once the first oxidative dehydrogenation step is eliminated, pentene oxidation proceeds readily towards maleic and phthalic anhydride formation. The use of 1-pentene in the feed stream caused a bulk reduction of the catalyst from V2O5 to VO2 in the presence of gas phase oxygen.