An electrochemical quartz crystal microbalance (EQCM) has been applied to a quantitative adsorption study of halide anions such as I-, Br-, Cl-, and F-, as well as the change in the surface excess of water directly related to the halide adsorption on a highly ordered Au(lll) electrode. It was found that the strength of halide adsorption decreases in the sequence I- > Br- > Cl- > F-. The potential dependent surface coverage of halide anions and the concomitant change of surface excess of water were determined. The maximum coverages of I-, Br-, and Cl- on Au(111) before the onset of the halide oxidation were 0.43, 0.56, and 0.53, respectively, which are in good agreement with reported values determined by other methods. The water molecules on the surface were found to be expelled by close-packed adsorption of halides. The adsorption behavior of halide anions has been discussed extensively. This work has demonstrated that the combination of EQCM with single crystal electrodes can yield a wealth of information on the surface process, especially when it is accompanied with adsorption of electrochemically inactive species.