A new method for the visualization of the concentration boundary layer is described. The technique involves the use of a dilute solution of an indicator which reacts with H+ formed on the membrane surface to form a coloured trace when the electrodialysis cell is operating above the limiting current density. The thickness of the concentration boundary layer determined by the visualization method agrees well with results obtained from limiting current density measurements and theoretical predictions. The visualization method proposed in this work can be used for understanding the transport taking place between a solid wall and a liquid in steady and unsteady flow processes.