When a @[email protected] flows, the concentrations of the @[email protected] 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 @[email protected] As the distance from the electrode surface increases, the @[email protected] is more and more nearly attained. The thickness of the reaction layer is the distance from the electrode surface beyond which deviations from the @[email protected] between C and B are taken to be negligibly small.