The electrochemical quartz crystal microbalance (EQCM) was used to study adsorption of pyridine on gold from aqueous and butanol solutions. It was found that the effect of specific adsorption on the frequency shift could not be explained in terms of a mass change in the adsorbed layer that rigidly coupled to the oscillating surface. In order to explain our observations we suggest a model, which accounts for two important features of adsorption at solid-electrolyte interfaces: (a) an adsorbate replaces a certain amount of solvent molecules from the interfacial layer, and (b) adsorption may lead to a finite slippage at the adsorbed layer-solution interface. Relationships between the slip length and the microscopic properties of the interfacial layer are discussed, and a dependence of the slip length on surface excesses is suggested. The treatment of experimental data makes it possible to determine the slip length as a function of surface excesses, for pyridine adsorption on gold from aqueous and butanol solutions.