Electrodeposition of silver was investigated using an impedance technique (separately excited, passive technique) electrochemical quartz crystal microbalance (EQCM) in a room-temperature ionic liquid. The mass changes during silver deposition and dissolution were observed with the current efficiencies of nearly 100% during potential sweep, constant potential step, and constant current step experiments. The product of the viscosity and density of the electrolyte near the electrode, eta(L)rho(L), can be estimated by the resonance resistance, which can be monitored simultaneously with the resonance frequency. The change in the eta(L)rho(L) value during silver deposition was consistent with the change in the calculated concentration of Ag(I) near the electrode. During the outer-sphere electron-transfer reaction between ferrocenium and ferrocene, no significant changes in the mass and the eta(L)rho(L) value were observed. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3223669] All rights reserved.