The selective oxidation of butan-2-one to diacetyl has been studied in the temperature range 200-380 degrees C over vanadium oxide and vanadium-phosphorus oxide catalysts. In addition to diacetyl, the principal reaction products detected were acetic acid, acetaldehyde, methyl vinyl ketone, propionaldehyde and carbon dioxide. Detailed steady state and transient kinetic analysis indicate that there are three distinct reaction pathways which lead to the observed product distribution. In the first of these diacetyl and methyl vinyl ketone are formed via a common intermediate, namely acetoin: CH(3)COCH2CH(3) --> CH3CO(CHOH)CH3 --> CH3COCOCH3 + CH3COCH=CH2. Evidence for this reaction route include the fact that acetoin was detected as a reaction intermediate in Temporal Analysis of Products (TAP) and when acetoin was fed to the reactor it was converted into diacetyl and methyl vinyl ketone. The second reaction pathway observed involved the oxidation of the enol form of methyl ethyl ketone with the formation of acetic acid and acetaldehyde This reaction predominated at high oxygen partial pressures and represented a significant route away from diacetyl formation in these conditions. The third reaction pathway observed was the decomposition of methyl ethyl ketone to two molecules of acetaldehyde via a diol intermediate: CH3COCH2CH3 --> CH3(CHOH)(CHOH)CH3 --> 2 CH3CHO. Evidence that a second pathway was involved in the formation of acetaldehyde emerged from the fact that the molar ratio of acetaldehyde to acetic acid was always greater than unity in spite of the fact that acetic acid was more stable than acetaldehyde in our reaction conditions. The second pathway was confirmed when propiophenone was fed to the reactor. Cleavage of the enol form of propiophenone should lead only to the formation of benzoic acid and never benzaldehyde. In our reaction conditions benzaldehyde was in fact observed in the reaction products, confirming that a molecule bearing the aldehyde functional group could form on the carbonyl side of the substrate.