This paper presents an application of electrical resistance tomography to the investigation of mixing processes at plant scale. An 8-plane 16-electrode ring sensor installed within a stirred tank with a 1.5 m inner diameter is described. Three-dimensional and nonstationary behaviour of mixing processes are illustrated by the images obtained simultaneously from eight axial levels along the tank height. The results illustrate air-core vortex detection, miscible fluid mixing and gas-liquid mixing processes.