Co-oxidation of p-xylene (PX) and p-toluic acid (p-TA) to terephthalic acid (TA) in water with Mn acetate catalyst has been studied. Mn acetate showed better catalytic activity than Co acetate and no synergism was observed between two catalysts. During the reaction. PX was transformed to p-TA while p-TA was converted to TA by a radical reaction. In the co-oxidation, radicals generated from facile PX-to-p-TA conversion would assist more difficult p-TA to TA conversion. Addition of CO2 in the oxygen promoted the TA formation and suppressed the CO2 formation from the combustion of reactants. Additional transition metals such as Ni and Ti also promoted the catalytic activity of Mn catalyst. Furthermore, a synergism was observed between added CO2 and the added transition metal. Finally, the yield of terephthalic acid reached about 95% with Mn + (Ti, Ni or Cr) catalyst system and CO2 addition. (C) 2009 Elsevier B.V. All rights reserved.