Small additions of the halide ions (F −, Cl −, Br −, I −) to 1N H 2SO 4 result in marked deviations of the current density/potential curves as well as the capacitance/potential curves from nickel, compared with those obtained in pure H 2SO 4. The different actions are explained by the varying tendency of the halide ions to specific adsorption at the nickel surface. The time-dependence of the capacitance demonstrates the existence of a two-stage process for the adsorption of iodide ions. In the presence of iodide ions the influence of their concentration was measured with respect to current density and capacitance.