When a @K03399@ flows, the concentrations of the @E01940@ B and its precursor C at very small distances from the electrode surface are influenced both by mass transfer and by the finite rate of establishment of the @C01023@. As the distance from the electrode surface increases, the @C01023@ is more and more nearly attained. The thickness of the reaction layer is the distance from the electrode surface beyond which deviations from the @C01023@ between C and B are taken to be negligibly small.