Abstract Photooxidation of 1,2,3-, 1,2,4-, and 1,3,5-trimethylbenzenes was carried out in the NOx–air system. Formation of glyoxal, methylglyoxal and biacetyl was observed in the cases of 1,2,3- and 1,2,4-trimethylbenzenes, and only methylglyoxal was observed from 1,3,5-trimethylbenzene. Yields of glyoxal, methylglyoxal and biacetyl were 7, 18, and 45% and 8, 37, and 11% of trimethylbenzene consumed in cases of 1,2,3- and 1,2,4-trimethylbenzenes, respectively. The reaction mechanism which was used to explain the yields of α-dicarbonyl compounds from toluene and o-, m-, p-xylenes in the preceding paper (part II) has been applied to explain these yields, and has been found to reproduce these yields well. Formation of 3-hexene-2,5-dione was observed only from 1,2,4-trimethylbenzene. The fractions of the ring-cleavage process in the total reaction were 70, 56, and 64% in cases of 1,2,3-, 1,2,4-, and 1,3,5-trimethylbenzenes, respectively. The yields of the ring-cleavage process are discussed.