The changes in surface energy of a gold electrode in deaerated 1.0 mol dm(-3) NaClO4 solutions with and without iodide ions were sensitively detected by using a piezoelectric technique. The potential of electrocapillary maximum or potential of zero charge (pzc) of gold electrode was evaluated from the piezoelectric signal curve. The addition of iodide ions shifted significantly pzc to the negative direction, indicating the strong contact adsorption of iodide ions. It was found from the iodide concentration dependence of pzc that the Esin-Markov relation held at pzc. The piezoelectric signals at the high potentials far from pzc suggested the possibility of structural change or phase change of iodine adlayer with sign reversal of total surface charge density.