The start-up of natural convection in an inhomogeneous magnetic field was studied in a OAM CuSO(4)solution between vertical electrodes. A Mach-Zehnder interferometer was used for the measurements of the concentration while the velocity field was determined by particle image velocimetry (PIV). The main phenomenon reported is the growth of a bubble-like zone of less-concentrated solution at the bottom of the cathode. This zone rises along the cathode if buoyancy is sufficiently strong to compete with the opposing Lorentz force. The results are discussed in terms of a scaling analysis. (c) 2008 Elsevier Ltd. All rights reserved.