The thermodynamics of halide-water and gold-halide-water systems are summarized in the form of potential-pH and activity-pH diagrams, calculated from recently published standard Gibbs energies of formation of the various species and phases considered. The stability of the gold-halide complexes increases in the order Cl < Br < 1, which is reflected in the pH above which Au(OH)3 (c) is predicted to form, though even the chloride complexes are stable at pH < 7. These diagrams enable predictions to be made of the behaviour of gold in halide electrolytes; chloride is used in industrial gold electrorefining, and interest is increasing in coupled leach-electrowin processes involving electrogenerated chlorine, bromine and iodine.