Diethyl 3,4-ethylenedioxythiophene-2,5-dicarboxylate was efficiently synthesized via the O-alkylation of disodium salt of diethyl 3,4-dihydroxythiophene-2,5-dicarboxylate with 1,2-dichloroethane over ionic type phase transfer catalysts, such as tetrabutyl ammonium bromide and benzyltriethyl ammonium chloride. The ionic type phase transfer catalysts showed higher catalytic activities than the nonionic type phase transfer catalysts, such as triethylamine, pyridine, 18-crown-6, and polyethylene glycol 400/600, in the O-alkylation reaction. The conversion of the disodium salt of more than 97% and the selectivity of diethyl 3,4-ethylenedioxythiophene-2,5-dicarboxylate of more than 98% were achieved when the O-alkylation reaction was synergistically catalyzed by tetrabutyl ammonium bromide and potassium iodide.