In this paper we show that a mixture of xylenes can be simultaneously oxidised in supercritical water (scH₂O) in a continuous mode to a mixture of the corresponding carboxylic acids in high combined yield, despite the differences in reactivity of the xylene isomers in conventional oxidation. The single phase environment in scH₂O together with the effect of higher temperatures should increase the reaction rate for each of these oxidation reactions and thus reduce the reactivity differences between the components of the C₈ refinery mixture. Such a process should lead to a considerable reduction in the overall energy input for the oxidation of xylenes. The process in scH₂O described here could simplify the downstream purification processes to a simple crystallization process. This is commercially important, because the purification process can be as expensive as the reaction producing the product. Furthermore, the oxidation of mixed xylenes could avoid the need not only for downstream purification, but also for the upstream separation of the xylene isomers. The use of high temperature water also offers significant cost advantages through enhanced energy recovery, due to a higher process temperature. Finally, the process totally eliminates the use of organic solvents.