The formation of mixed self-assembled monolayers (SAMs) of different 4＇-substituted-4-mercaptobiphenyl (with X = CH3, CF3, and SCH3) on gold, from different solvents, has been investigated. Monolayer thickness values, estimated by ellipsometry, contact angle measurements, and external reflection absorption infrared (ERFTIR) spectroscopy confirms that >90% of the monolayer is formed after 1 h of adsorption. The dielectric constant of the solvent, the permanent dipole moment of the substituted mercaptobiphenyl, their adsorption and desorption kinetics, and their intermolecular dipole-dipole interactions in the SAM were all found to play a crucial role in the final composition of the mixed SAMs.