The thermodynamic properties of self-assembled monolayer (SAM) films composed of long and short alkanethiol chains are predicted from the statistical mechanical calculation and the dynamic Monte Carlo simulation, analogous to the Ising spin model in conjunction with interaction energy between the long and the short chains. Numbers of pairs of long-short chains are obtained for the interaction energy of the long-short chain and molar fractions. These relations are compared with the experimental result of binary SAMs, in which a small voltammetric wave appeared at a potential between two potentials of the desorbed reduction waves of long and short chains (J. Electroanal. Chem. 444 (1998) 113). The small wave can be ascribed to a new redox state generated by the interaction between the short and the long chains. The comparison estimates the interaction energy between the closest neighboring methylene groups to be 7.4 0 mol(-1) at 25 degreesC. A pair of long-short chains is more unstable than pairs of long-long and short-short chains, and hence the long and short chains produce their own macroscopic phases. Monte Carlo simulation supports the appearance of the phase domains.