The evolution of the velocity boundary layer during the initial phase of copper electrolysis under the influence of a magnetic field is studied by using particle image velocimetry and a novel laser Doppler velocity profile sensor. With this new sensor, time-resolved velocity measurements within 400 mu m of a vertically aligned cathode in an aqueous 0.05 M CuSO4-solution are presented. In this way, the complex interaction of Lorentz force and opposing buoyancy-driven convection was studied by measuring the resulting velocity profile inside the concentration boundary layer with a spatial resolution of 15 mu m. It is shown that the Lorentz force-driven convection only dominates the velocity boundary layer during the early phase of electrolysis and induces a linear velocity profile near the cathode. The linear relationship between the velocity gradient and Lorentz force is determined. With the onset of the opposing buoyancy-driven convection at the cathode, a duplex structure of the boundary layer appears. Its characteristic quantities, given by the horizontal distances, delta(max) and delta(nu=0), where the velocity reaches the maximum and where it is equal to zero, remain nearly unchanged, while the maximum velocity, nu(max), in spite of the counteracting Lorentz force, increases faster as compared to pure natural convection, depending on the current density. (C) 2011 Elsevier Ltd. All rights reserved.