With fast data acquisition and good spatial discrimination, electrical resistance tomography (ERT) provides a non-invasive and non-intrusive technique to interrogate, in three dimensions, the concentration fields inside a typical stirred vessel. A newly refurbished vertically assembled 16-sensor eight-ring electrode array has been used to image the full volume of a 23001 pilot-scale vessel for single-feed semi-batch operation. Images reconstructed from the raw ERT data are represented first as pixel conductivity distributions and secondly as time incremented three-dimensional solid-body colour-scaled isosurfaces thereby providing a five-dimensional representation of the mixing process. When operating in this single-feed semi-batch mode, the ERT system can clearly distinguish the higher conductivity feed plume. The ERT also images the subsequent mixing in the vessel after the semi-batch feed addition has stopped, thus additionally characterizing the role of macro-mixing in achieving final homogeneity after a period of semi-batch operation. The results to be presented are extremely useful for the validation of CFD predictions of mixing behaviour.