The adsorption of iodide at a Au(111) single crystal electrode has been investigated quantitatively using chronocoulometry. By analyzing the charge density data thermodynamically, the following parameters were determined: the Gibbs excess, Gibbs energy of adsorption, the number of electrons flowing to the interface per one adsorbed iodide ion at a constant electrode potential (electrosorption valency), and at a constant chemical potential. The thermodynamic data for iodide adsorption were compared to the results for bromide and chloride adsorption. All the three halides form a chemisorption bond with the gold surface. The bond is quite polar at the negatively charged surface, however, its polarity drops significantly at the positively charged surface. At low charge densities and coverages. the bond polarity is determined by the ability of free electrons to screen the dipole formed by the adsorbed anion and its image charge in the metal. At high charge densities and coverages, the chemisorption bond has a predominantly covalent character. The strength of the halide adsorption and the covalent character of the chemisorption bond increase progressively by moving from chloride to iodide.