PURPOSE: To obtain the subject substance according to separation of a solid from a liquid by oxidatively dimerizing 2,6-xylenol in an alkaline aqueous sol vent containing a surfactant in the presence of a metal catalyst, regulating the reaction solution to acidic conditions, then adding a lower alcohol to the reaction product, mixing and stirring the resultant mixture.

CONSTITUTION: 2,6-Xylenol is oxidatively dimerized in the presence of a metal catalyst (e.g. a copper compound) in an alkaline aqueous solvent (the alkaline substance is preferably a boron compound such as sodium borate) containing a surfactant (e.g. an alkylsulfonate) at 50-100°C. The resultant reaction solution is then regulated to pH≤7.0 with sulfuric acid, etc., and a 1-4C alcohol is subse quently added to the reaction product, mixed, stirred in a medium at (1/9)-(9/1) weight ratio of water/alcohol and cooled to carry out separation of a solid from a liquid. Thereby, the subject high-purity substance is obtained. The subject substance is useful as a raw material for polyesters, polycarbonates, epoxy resins, etc., and raw material, etc., for stabilizers or plasticizers of petroleum products.