Trimellitic acid is produced by oxidation of pseudocumene in acetic acid at temperatures between 130 and 240° C. in the presence of a catalyst composition containing cobalt, manganese and bromine. The process comprises the stages of: (i) simultaneously feeding pseudocumene and air or an oxygen-containing gas into a solution containing acetic acid and the catalyst composition, at an addition rate resulting in an oxygen concentration in the off-gas of less than 8 vol %, and at a temperature and a pressure sufficient to result in 5 to 25 mol % of the total amount of oxygen required to oxidize the pseudocumene to trimellitic acid being consumed, and (ii) feeding air or an oxygen-containing gas into the reaction mixture obtained in stage (i) until essentially all of the pseudocumene has been consumed and a molar yield of trimellitic acid of at least 90% has been obtained. There is no supplementation of catalyst after stage (i).