Mixed adlayers; of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine copper(II) (CuOEP) and cobalt(II) phthalocyanine (CoPc) were prepared by immersing Au(111) substrate in a benzene solution containing CuOEP and CoPc molecules, and they were investigated in 0.1 M HClO4 by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy (STM). The composition of the mixed adlayer consisting of CuOEP and CoPc molecules was found to vary depending on the immersion time. CoPc molecules displaced CuOEP molecules during the modification process with increasing immersion time, and the CuOEP molecules were completely replaced with CoPc molecules in the mixed solution after a long modification time. The two-component adlayer consisting of CuOEP and CoPc, which has a structure with the constituent molecules arranged alternately, was found to form either a p(9 x 3root7R - 40.9degrees) or a p(9 x 3root7R - 19.1degrees) structure, each involving two molecules on the Au(111) surface. The surface mobility and the molecular reorganization of CuOEP and CoPc were accelerated by modulation of the electrode potential. Different surface structures were produced at different electrode potentials, and hence potential modulation should allow a precisely controllable phase separation to take place in aqueous HClO4.