The adsorption process of iodide on a highly ordered Au(111) electrode has been investigated using an electrochemical quartz crystal microbalance. Considering that the change of electric charge Delta Q is due to the adsorption or desorption of iodide ions on the surface, but the change of electrode mass Delta W is due to the adsorption or desorption of iodide ions and water molecules on the surface, the surface concentration of iodide ions and the change of surface excess of water on the Au(111) surface have been determined quantitatively. The coverages of Au(111) with iodide as a function of the electrode potential were quite consistent with those estimated from the scanning tunneling microscope image or using surface X-ray scattering. It was found that the surface excess of water changed with electrode potential during the adsorption of iodide on Au(111), showing a maximum around -0.20 V.