PURPOSE: To prepare dimethylbinaphthyls useful as a chemical raw material, etc., in an electrically high yield, high purity and high yield by dimerizing methylnaphthalene in the presence of a specific electrolyte salt mixture by an electrochemical oxidation.

CONSTITUTION: 2-Methylnaphthalene is electrochemically oxidized in the presence of a mixture of acetonitrile, water and an electrolyte salt (e.g. trifluoromethanesulfonate), additionally containing one or more other components immiscible or partially miscible with water, at a temperature of 20-50°C to carry out dimerization, and the obtained reaction mixture is rectified under a reduced pressure to provide the objective 2,2'-dimethyl-1,1'-binaphthyl and 2,7'-dimethyl-1,1'-binaphyl.