The promotional effects of CO2 and Ni were studied on Co/Mn/Br catalyst in the liquid-phase oxidation of p-xylene to terephthalic acid using molecular oxygen as an oxidant and acetic acid as a solvent individually as well as in combination. The enhanced activity of Co/Mn/Br catalyst was observed on both CO2 and Ni promoters independently and also in combination. The activity enhancement in the combination of CO2 and Ni promoters on this catalyst is found to be remarkable.