We have demonstrated that different methylaromatic compounds [1,4-dimethylbenzene (p-xylene), 1,3-dimethylbenzene (m-xylene), 1,2-dimethylbenzene (o-xylene), 1,3,5-trimethylbenzene (mesitylene) and 1,2,4-trimethylbenzene (pseudocumene)] can be aerobically oxidized in supercritical water (scH2O) using manganese(II) bromide as catalyst to give corresponding carboxylic acids in the continuous mode over a sustained period of time in good yield. No partially oxidized intermediates (i.e., toluic acid and benzaldehydes) were detected for the dimethylbenzenes and mesitylene reactions. By fine tuning pressure and temperature, scH2O becomes a solvent with physical properties suitable for single-phase oxidation since both organic substrate and oxygen are soluble in scH2O. There is a strong structural similarity of metal/bromide coordination compounds in the active oxidation solvents (acetic acid and scH2O) which does not exist in the much less active H2O at lower temperatures. This may account for the successful catalysis of the reactions reported herein. Aromatic acids produced by the loss of one methyl group occurred in all of these reactions, i.e., 3–6% benzoic acid formed during the oxidation of the dimethylbenzenes. Part of this loss is thought to be due to thermal decarboxylation. The thermal decarboxylation process is monitored via Raman spectroscopy.