PURPOSE: Pseudocumene is oxidized with O2 using a catalyst that is composed of a specific concentration of Co-Mn-Br system, thus shows increased activity and less deactivation to produce the titled compound used as a raw material of heat-resistant plasticizer and high polymers of various kinds in high yield.

CONSTITUTION: Pseudocumene is oxidized with an oxygen-containing gas in a solvent of acetic acid in the presence of a Co-Mn-Br system catalyst to form trimellitic acid wherein the amount of Co in the catalyst is 0.01W0.2wt% based on the acetic acid solvent, Mn is 1W20wt% based on Co and Br is 6W20times the weight of Co. Preferably the reaction is conducted by supplying pseudocumene continuously to the reaction system and using the catalyst in the above range. Then, in the system where the feeding is stopped, the reaction mixture is brought into contact with an oxygencontaining gas again at a temperature not more than 10°C lower than the reaction temperature.