Process for producing trimellitic acid

A process for producing trimellitic acid by liquid phase air oxidation of pseudocumene in an acetic acid reaction medium in the presence of a catalyst consisting of of catalytic forms of cobalt, manganese and bromine, which comprises conducting the reaction at two stages comprising a first stage of reaction at a temperature of 110.degree. to 170.degree. C. and a second stage of reaction at a temperature of 180.degree. to 240.degree. C., while maintaining a reaction pressure under a condition of the following formula throughout the reaction:

• 1.20Psolv.ltoreq.P.ltoreq.2.20Psolv
• wherein P represents a reaction pressure as absolute pressure and Psolv represents a total vapor pressure of acetic acid and water present in the reaction system at a given reaction temperature and the first stage of reaction at a lower temperature is continued until the amount of unreacted pseudocumene in the reaction solution becomes 10% or less on the basis of charged pseudocumene.