The promoting effect of acetoxime ((CH 3 ) 2 C NOH) addition on the catalytic performances of FeCl 2 catalyst for catalyzing the selective oxidation of 2,4,6-trimethylphenol (2,4,6-TMP) to 3,5-dimethyl-4-hydroxybenzaldehyde (DMHB) was studied with a high-pressure batch reactor in the temperature range of 50-90 โข C. A marked increase of the reaction rate upon the addition of acetoxime was observed. The extent of activity improvement increased with the reaction temperature and reached a six-fold increase of the reaction rate at 90 โข C. The maximum DMHB yield obtained with the FeCl 2 -acetoxime catalyst was 80%, which was higher than that (75%) obtained with FeCl 2 alone. The rate of 2,4,6-TMP disappearance was found to exhibit zero-order dependence of the TMP concentration up to 76% conversion. A reaction mechanism, similar to that proposed previously for the oxidation of substituted toluene by cobalt(III) acetate in acetic acid solution, was employed successfully to explain the experimental results obtained. The activity improvement with the addition of acetoxime was ascribed to its better electron-donating ability, which enhanced the dissociation rate of the intermediate complex.