Supercritical (sc) fluids are considered to be a promising alternative to toxic organic solvents in a variety of processes. In this work, a study of the synthesis of terephthalic acid (TPA) from p-xylene (PX) using supercritical carbon dioxide (scCO(2)) as a reaction medium was performed. The effects of various process parameters were investigated, including temperature, residence time, and catalyst loading. The maximum molar yield of TPA that was achieved was less than 35%. The oxidation reaction for producing TPA was unfavorable under most conditions, which is likely due to the poor solubility of the CoBr2 catalyst in scCO(2). This was able to be improved by the addition of a small amount of acetic acid or water to the reaction mixture, resulting in improved PX conversion and TPA yield. Furthermore, it was found that the use of an organic cobalt alternative could improve the yield.