Interfacial proton transfer reactions of pure 3-mercaptopropionic acid (MPA) and ethanethiol (EtSH) mixed self-assembled monolayers (SAMs) have been studied using the faradaic impedance titration method. The Fe(CN)(6)(3-) is used to probe the charge development of the terminal -COOH group of the monolayers. The charge-transfer resistance (R-ct) is measured with the monolayer composition, adsorption coverage, and the ionic strength of pH solution. The surface pK determined for the pure MPA is 6.0 at a monolayer coverage and 0.1 M ionic strength. The in-plane electrostatic force effect, which causes a broadening of the titration curves, on the surface pK for the MPA SAM is not significant at an ionic strength over 0.1 M. The pK value for the pure MPA SAM shifts negatively as the surface coverage decreases, indicating that the in-plane interactions between acids and the hydrophobicity surrounding acids decrease at the same time. When the pure MPA SAM is compared with the EtSH mixed SAM, the surface pK of the mixed SAM is larger than that of the pure SAM. Such positive pK shifts are more pronounced at the pure MPA SAM with a lower coverage. This implies that the pK shifts of sparsely adsorbed acidthiol SAM are more sensitive to the introduction of hydrophobicity than to the decrease of the in-plane interactions.