Abstract: In the production of phthalic acid by oxidizing, in a liquid phase, tolualdehyde with molecular oxygen or a gas containing the molecular oxygen in the presence of heavy metal salt containing a Mn salt, especially both Mn and Co salts, and a bromine compound, using a lower aliphatic monocarboxylic acid as a reaction solvent, a blackening of phthalic acid is effectively prevented by conducting the oxidation of tolualdehyde in the presence of an alkylbenzene in an amount satisfying the following formula (1);M.ltoreq.-0.15A.sup.2 +15A+40 (1)wherein M is a Mn atom concentration in.ppm by weight on the basis of the reaction solution, and A is an amount in % by weight of the alkylbenzene added on the basis of the total of tolualdehyde and said alkylbenzene, and is in a range of from more than 0 to 50 inclusive.

Abstract: Terephthalic acid is produced by oxidizing, in a liquid phase, p-tolualdehyde with a gas containing molecular oxygen, for example, air, in the presence of a catalyst consisting each of (1) a manganese compound, (2) a cobalt compound, (3) a bromine compound, and (4) at least one compound selected from the group consisting of chromium compounds, iron compounds, nickel compounds and compounds of metallic elements belonging to the lanthanide series, using a lower aliphatic monocarboxylic acid as a solvent.