The electrochemical quartz crystal microbalance was used to obtain information on the structure of the metal/solution interface. In our previous studies, pyridine was used as a model system, and its adsorption on gold and silver were compared. In the present work this study was extended to determine the effect of the solvent, employing the same adsorbate as a probe. Adsorption of pyridine on gold was measured, using n-butanol as the solvent. In addition, the adsorption of butanol from aqueous solutions on gold was determined. In both cases the frequency increases with increasing surface excess. This can be partially explained by the replacement of a number of water molecules from the surface, by molecules of adsorbate. However, quantitative evaluation of the results shows that the mass effect by itself is not sufficient to account for the observed frequency shift. The configuration of the solvent at the interface, and its ability to form hydrogen bonds with adjacent molecules in the bulk, influences the frequency shift. Moreover, an organic molecule adsorbed on the surface changes the nature of the interactions between the vibrating metal and the solvent, giving rise to a change of frequency. These contributions to the observed frequency shift call also be explained in the framework of a model postulating the existence of a very thin film adjacent to the surface, in which the viscosity and density differ significantly from their bulk values.