For the first time, the capability of three-dimensional EIT to monitor reactive systems under realistic process conditions and scale has been demonstrated. The semi-batch precipitation of barium sulphate from barium chloride and sodium sulphate has been investigated in a 2001, semi-tech scale, stir-red tank reactor. Experiments have been performed using both Rushton and pitched blade turbines under a range of agitation rates with a surface feed of barium chloride. The repeatability of EIT in monitoring the reaction progress has been illustrated. The visualisation of plume structure and evolution during the semi-batch reaction has been examined in 3D and linked to the mixing curve. The quantification of the effect of changes in reactor level on reconstructed images has also been examined suggesting this is a limitation to the commonly used finite element approach, as only a 2% change can cause significant effects. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.