Viologens possessing long alkyl chains are condensed to monolayers of their radical cations upon one-electron reduction on a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode in contact with not only one-component viologen aqueous solutions but also binary mixtures. For three types of binary mixtures of viologen including ten different combinations, the condensed phase formation processes were described using the results of voltammetric measurements. First, in the case of the binary mixture of symmetric di-alkyl viologens (dA) with different chain lengths, two-dimensional (2D) phase separation into two domains took place in the course of cathodic potential scan in a certain range of the solution molar fraction when the chain length difference is two methylene units or more. Second. in the case of the mixture of a dA and its both-end carboxylated derivative (bis-carboxylated viologen: bC) either pure bC condensed phase or dA + bC well-mixed phase covered the entire Surface area. Third, in the case of the mixture of dA and 15C with far different chain lengths, the molar fraction dependent condensation processes took place while bC always dominates the phase formation behavior. The implications of these results were discussed as typical 21) molecular nano-organization processes in the terms of homo- and hetero-intermolecular interactions on the HOPG surface. (C) 2008 Elsevier B.V. All rights reserved.