Gas phase oxidation of alpha- and beta-isophorone to ketoisophorone (2,6,6-trimethyl-cyclohex-2-ene-1,4-dione) has been investigated over various catalysts. Good combined selectivities to allylic oxidation products ketoisophorone and formylisophorone (3-formyl-5,5-dimethyl-2-cyclohexen-1-one) of ca. 75% are obtained at 17% conversion of alpha-isophorone, with 90 vol.% oxygen in the gas feed and in the temperature range 200-250 degrees C. The major byproduct is beta-isophorone (188) which can be recycled together with unconverted reactant. Studies of unsupported vanadia and magnesia or lithium phosphate-modified pumice supported vanadia highlight the influence of support in determining selectivity. Vanadia-based catalysts and other systems investigated including 10 wt.% chromia/gamma-alumina, bismuth molybdate, vanadium phosphate and 5 wt.% silver/gamma-alumina all deactivate under continuous operation. The aerobic gas phase oxidation of isophorone is discussed in context with competing gas phase isomerization and rearrangement reactions.