Studies on electrochemical polymerization of pyrrole in the presence of thiophene are given for preparation of polypyrrole (PPy)/polythiophene (PTh) double layers. At a constant current, only pyrrole is electrochemically polymerized in the electrolytic solution containing pyrrole and thiophene in the present experiments. The cyclic voltammograms on pyrrole/ thiophene mixtures are similar to that on pyrrole. Preparation of PPy/PTh double layers is carried out by electrochemical polymerization of pyrrole in the presence of thiophene by adding pyrrole and by lowering voltage immediately after electrochemical homopolymerization of thiophene. The resulting double layers show good rectification characters dependent on thickness.