Liquid-phase catalytic oxidation of p-xylene to terephthalic acid was performed at 150-210 degrees C over Co-Mn- Br catalyst system. There was an interesting synergistic effect of cobalt and manganese catalyst. In the previous papers (Ind. Eng. Chem. Res. 2005,44, 261- 266; 4518-4522; 7756-7760), a lumped kinetic scheme and a fractional kinetic model for the liquid-phase oxidation of p-xylene were proposed and tested, and the effects of catalyst concentration, water content, and guanidine catalyst additive were investigated. In this paper, the synergistic effect of cobalt and manganese on the oxidation kinetics was studied. Experiments of several levels of Co/Mn ratio and temperature were carried out in a semibatch oxidation reactor where the gas and liquid phases were well-mixed. The results showed that the variation of the Co/Mn ratio in the catalyst did affect the activity and selectivity of the catalyst system. For the main reaction, there existed an optimum Co/Mn ratio at which the rates of the latter two slower oxidation steps of p-toluic acid and 4-carboxybenzaldehyde reached the maximum, and the optimum Co/Mn ratio decreased with the increase of reaction temperature. However, the burning side reaction of reactant and solvent due to carbon dioxide and carbon monoxide formation increased remarkably with the increase of Co/Mn ratio. A possible mechanism that the optimum Co/Mn ratio presents in the oxidation was proposed eventually.