Impedance measurements were taken at a Pt(111) and a polycrystalline electrode in alkaline electrolyte. The effect of iodide in the solution was also studied. The rate of iodide adsorption was too fast to be determined. In iodine-free solution, the rate of hydrogen adsorption is higher on Pt(111) than on the polycrystalline electrode by one order of magnitude. Whereas iodide does not influence the rate of hydrogen adsorption in the case of the polycrystalline electrode, it causes a marked decrease in this rate on Pt(111) even at potentials where only traces of iodine are adsorbed.