Mixed self-assembled monolayers (SAMs) of 4＇-methyl-4-mercaptobiphenyl and 4＇-hydroxy-4-mercaptobiphenyl were prepared on gold (111) surfaces. The composition of the mixed SAMs could be precisely determined by external reflection Fourier transform infrared (ER-FTIR) spectroscopy and showed direct correlation with their wetting properties. Contact angles of water, glycerol and diiodomethane showed small hysteresis, and their values did not change after 1 month, suggesting that these mixed SAMs are stable and can be utilized as model systems for the study of surface phenomena.