Abstract: A process for producing trimellitic acid by oxidizing pseudocumene with a molecular oxygen-containing gas in an acetic acid solvent in the presence of a catalyst comprising one or more cobalt compounds, one or more manganese compounds, and one or more bromine compounds, wherein the total amount of the cobalt compound is in the range of 0.01-1.0% by weight of the acetic acid solvent based on the weight of cobalt metal, the total amount of the manganese compound is in the range of 0.01-0.1% by weight of the acetic acid solvent based on the weight of manganese metal, the total amount of the bromine compound is in the range of 0.01-2.0% by weight of the acetic acid solvent based on the weight of bromine atoms, with the proviso that the atomic ratio of bromine to cobalt and manganese [Br/(Co+Mn)] is in the range of 2.51-2.99, which process comprises at least two reaction-stages comprising the preceding reaction-stage in which the reaction temperature is maintained in the range of 110.degree. to 180.degree. C.