Mixed self-assembled monolayers (SAMs) of 4＇-nitro-4-mercaptobiphenyl (I) and 4＇-(dimethylamino)-4-mercaptobiphenyl (III) have been compared with that of 4＇-nitro-4-mercaptobiphenyl (I) and 4＇-methylmercapto-4-mercaptobiphenyl (II). The composition of the mixed SAMs was determined by external reflection Fourier transform infrared (ER-FTIR) spectroscopy and showed that both the magnitude and direction of dipole moment determined the final composition of mixed SAMs. In toluene, with a dielectric constant of 2.4, a significant enhancement dipolar interaction is observed. The composition of mixed SAMs of I and II showed a plateau at 40% surface concentration of I. These results can be explained by simple calculation based on the Hammett equation. The experimental results indicate that the equilibrium concentration of the two components in the mixed SAM(I/II), in a nonpolar solvent, is driven by the formation of a two-dimensional assembly with zero net dipole moment.