A method of producing naphthalenedicarboxylic acids by the oxidation of dialkyl-substituted naphthalene with a gas containing molecular oxygen under liquid phase conditions in an organic solvent and in the presence of a catalyst comprising copper and bromine, or a catalyst comprising copper, bromine and at least one kind of element/compound selected from the group of consisting of amine compounds and heavy metallic elements which are vanadium, manganese, iron, cobalt, nickel, palladium and cerium. And a method of producing diaryldicarboxylic acids by the oxidation of dialkyl-substituted diaryl compounds with a gas containing molecular oxygen in an organic solvent and in the presence of the same catalyst. These methods permit high yields of naphthalenedicarboxylic acids of high purity and of diaryldicarboxylic acids of high purity with the use of small amounts of catalyst.