Catalytic air-oxidation of 1, 2, 4, 5-tetraisopropylbenzene (TIPB) to pyromellitic dianhydride (PMDA) in the gas phase has been studied. Among catalysts tested, V2O5-P2O5-alundum catalyst (10:1.5:100 in weight) showed the highest yield of PMDA. As compared with durene, TIPB was considerably difficult to oxidize and was converted to PMDA in lower yield on oxidation with the V-P-O catalyst.In oxidation of toluene and cumene on the same catalyst, however, cumene was more easily oxidized and gave higher yield of benzoic acid than toluene. Therefore, it may be suggested that the low oxidizability of TIPB is due to the steric hindrance of the adjacent isopropyl group in oxidation of isopropyl groups and the lower yield of PMDA in the case of the oxidation of TIPB is due to a kind of steric hindrance to apparently simultaneous oxidation of two adjacent alkyl groups to anhydride structure.5-Isopropylpseudocumene, 2, 5-diisopropyl-p-xylene and 4, 6-diisopropyl-m-xylene were oxidized to PMDA in yields higher than that obtained from TIPB but in yields lower than that from durene.