Piezoelectric quartz crystals, modified by the spontaneous adsorption of either 4-aminothiophenol or octadecanethiol, were utilized to investigate the selective interactions that exist between solutes and the surface modification layer. It is found that the exposed amine functionality of the 4-aminothiophenol monolayer increases the sensitivity of the quartz crystal microbalance to phenols relative to the response found at either a crystal modified with an octadecanethiol monolayer or an unmodified quartz crystal. Protonation of the amino group decreases the sensitivity of the 4-aminothiophenol modified crystal toward the phenol solutes, to a level approximately the same as seen for the octadecanethiol monolayer and the unmodified quartz crystal. The response of the three quartz interfaces studied to structurally similar aniline molecules ia nearly the same for each interface and is approximately the same as observed for the phenols at both the octadecanethiol modified interface and the unmodified quartz crystal. These results indicate that a selective interaction between the solute and monolayer leads to increased sensitivity of the quartz crystal microbalance to the solute. For the phenols, the principal mode of interaction at the interface likely involves hydrogen bonding with the basic 4-aminothiophenol. Theoretical fit of the adsorption data to Langmuir isotherms provides a mechanism to determine the free energy of adsorption for the phenols at the 4-aminothiophenol modified quartz crystal. The free energy of adsorption was found to be -16.2 ± 0.5 kJ/mol for 4-tert-butylphenol, -14.4 ± 0.6 kJ/mol for 4-isopropylphenol, and -14.8 ± 0.8 kJ/mol for 4-ethylphenol when interacting with a 4-aminothiophenol modifed surface.