The response of the electrochemical quartz crystal microbalance (EQCM) was studied in 0.1 M HQlO(4), KNO3, and ROH. The gold coating on the crystal served as the working electrode, and the frequency was determined as a function potential in the double-layer region. Frequency shifts up to -5 Hz were observed, even though the ions of the electrolytes employed are not specifically adsorbed and there is no faradaic reaction which could lead to the formation of adsorbed species through charge-transfer. The added weight which would cause a similar shift in frequency in our experimental setup is ca. 60 ng/cm(2), which is equivalent to about a monolayer of chlorine atoms adsorbed on the surface. Thus, elucidation of the origin of this effect is essential for the proper use of the EQCM in the submonolayer region. The effects observed are due to the surface excess of ions in the diffuse double layer. A model was developed in which the liquid in a thin layer near the surface has a higher viscosity than in the bulk, because of the high concentration of ions and the high electrical field in this region. The value of this viscosity can neither be calculated independently nor measured experimentally, since the liquid in this region is charged (electroneutrality is maintained across the interface but not on each side separately). Using the viscosity of the film as a parameter we were able model the experimental behavior.