The catalytic oxidation of 2,4,6-trimethylphenol to 3,5-dimethyl-4-hydroxybenzaldehyde (DMHB) was studied in alcohols under 10 kg/cm(2) pure oxygen pressure. Five iron halides were used as the catalysts, it was found that the oxidation activity and the DMHB yield decreased in the order FeCl2 > FeBr3 > FeCl3 much greater than FeF2 > FeI2. The solvent had significant effect on the FeCl2 catalyst performances, but the solvent effect was opposite to that observed earlier for a CuCl2 catalyst. Among alcohols with 1-6 carbon numbers, methanol was the best solvent for FeCl2, but it was reported earlier to be the worst solvent for CuCl2. In the temperature range of 70-110degreesC, the maximum DMHB yield obtained with FeCl2 was around 75% (this yield was higher than that obtained with CuCl2), which occurred at the 2,4,6-TMP conversion of around 93%. Further increase of 2,4,6-TMP conversion resulted in the rapid increase of the by-product (2,6-dimethyl-p-benzoquinone, DMBQ), which suggested that most of the DMBQ came from the oxidation of the desired product. An electron transfer (from 2,4,6-TMP to iron (III)) mechanism involving the intermediacy of an aromatic radical cation was proposed for the oxidation of 2,4,6-TMP to DMHB. Based on the mechanism, the effects of solvents and halide ligands were explained. (C) 2004 Elsevier B.V. All rights reserved.