PURPOSE: To obtain aromatic polycarboxylic acids with high quality in high yield, by two steps of oxidation of a polyalkyl aromatic hydrocarbon at a relatively low temperature under a relatively high oxygen partial pressure, while avoiding formation of by-products and impurities.

CONSTITUTION: A mixture of a solvent comprising water and a 2-6C aliphatic monocarboxylic acid, an alkyl aromatic hydrocarbon having two to four of 1-5C alkyl groups and a Br-Co-Mn catalyst is placed in a first reactor. Oxygen- containing gas is introduced to the reactor in a proportion of 1.5-2.7 moles per alkyl group. The alkyl aromatic hydrocarbon is oxidised to a monocarboxylic acid and a monoaldehyde at 200-350°C and oxygen partial pressure of ≥0.1 kg/cm2 under a pressure of 10-34 kg/cm2 in the reactor. A portion of the liquid phase in the first reactor is transferred to a second reactor, and oxygen is introduced at a ratio of 1.5-2.7 moles per alkyl and the oxidation is carried out 275-450°F and oxygen partial pressure of ≥0.1 kg/cm2 under a pressure of 5-30 kg/cm2, thereby to given an aromatic polycarboxylic acid. The reaction temperature and pressure in each reactor are controlled by means of a liquid phase heat exchanger.