The diffusion layer thickness, δ, was measured as a function of time by a Mach‐Zehnder interferometer on a vertical cathode in solution with and without an excess of at current densities, i, up to 100 mA/cm2. The thickness δ increased linearly with t½ up to the start of hydrogen gas evolution in solutions with and without ; in the presence of tenfold excess of ,δ rapidly increased just before the gas evolution. At a constant cathodic charge, log δ decreased linearly with increasing log i in both solutions and δ is larger in pure solution than in the presence of excess of .