PURPOSE: To obtain the titled compound useful as fiber, film, plasticizer, resin, etc., in high yield, by contacting 4,4'-diethyldiphenyl with a gas containing molecular oxygen in liquid phase in the presence of methanol.

CONSTITUTION: The objective compound can be produced by oxidizing 4,4'- diethyldiphenyl with a molecular oxygen-containing gas using a catalyst containing at least a manganese compound (preferably a mixture with a cobalt compound or a nickel compound) in the absence of solvent or in a solvent exhibiting liquid state under the reaction condition (e.g. carboxylic acid ester) in the presence of methanol under total pressure of atmospheric pressure W 250kg/cm2G and an oxygen partial pressure of 0.2W50kg/cm2G, preferably 0.2W10 kg/cm2G at 200W300°C.