The focus of the present paper is the evolution of the complete concentration field during galvanostatic electrolysis of a 0.1 M CuSO4 solution between vertical electrodes. For this purpose a cubic cell with a side length of 10 mm is used which is analysed in a Mach-zehnder interferometer. The whole electrolysis process can be divided into three distinct regimes. The quasi-steady state in which horizontal diffusion is balanced by vertical advection is sandwiched by an initial and a final transient. The latter results from the small-aspect-ratio geometry in which the so-called intrusion layers, formed as a discharging product of the vertical concentration boundary layers, play a dominant role. They are responsible for deviations from the classical scaling of the concentration boundary layer thickness. Furthermore, due to the different rate-controlling steps at cathode and anode, a strong concentration asymmetry between top and bottom of the cell occurs. The experimental results are compared with a prediction of the scaling analysis derived in the appendix. (C) 2007 Elsevier B.V. All rights reserved.