In a method of producing pyromellitic acid by liquid-phase oxidizing
2,4,5-trimethylbenzaldehyde in a water solvent with molecular oxygen, a
recrystallization mother liquor separated in a recrystallization step is
recycled to the oxidation step after a part of the water solvent is
removed. With this method, the loss of pyromellitic acid and the
oxidation catalyst can be minimized and the burden of discharging the
waste water is reduced without lowering the efficiency of the
liquid-phase oxidation reaction.