The copper(II) complex of the water soluble ligand 1,4,7-tricarboxymethyl-1,4,7-triazacyclononane has been employed as a catalyst for the oxidative polymerization of 2,6-dimethylphenol using alkaline water as the solvent. With this catalytic system, the polymer, poly(2,6-dimethyl-1,4-phenylene oxide), was successfully obtained with suppression of the formation of the quinone by-product, 4-(3,5-dimethyl-4-oxo-2,5-cyclo-hexadienylidene)-2,6-dimethyl-2,5-cyclohexadienone, during the polymerization process.