An 8-plane 16-sensor ring electrical resistance tomography (ERT) system was designed and installed within a 1.5-m-dia. plant-scale stirred vessel. Results are reported for the unsteady dynamic macromixing of miscible liquids following a pulse addition of brine tracer. Acquired data, after reconstruction, is conveniently represented as a stack of 2-D tomograms and 3-D solid body isosurfaces, thereby providing an equivalent of medical whole-body scanning for a chemical process unit. The results demonstrate the ability of ERT to distinguish the different mixing patterns from two types of impeller in 3-D and to monitor plant-scale miscible liquid mixing processes experimentally with good resolution in both space and time.