Electrical resistance tomography (ERT) provides a noninvasive and nonintrusive technique for the full 3-D interrogation of a typical stirred vessel reactor up to a spatial discrimination of approximately 2,500 derived data points. Some recent ERT results are presented for a newly refurbished 8 ring 16-sensor 2.3 m(3) pilot scale vessel, detecting the conductivity changes throughout the whole volume of the vessel during the semi-batch precipitation of barium sulfate. Fast timewise acquisition at video frame rates provides an augmented-reality visualization of the precipitation process. Time evolving images created by solid-body iso-surfaces, which use color to denote conductivity, are presented during the proceeding reaction. These images provide a 5-D representation of the conductivity fields within the stirred vessel reactor. The conductivity (or pixel) distributions during the precipitation process provide a real-time measurement of the homogeneity within the vessel. The timewise conductivity profiles, over the whole of the vessel during the precipitation, indicate three very distinctive regions reflecting the complex reaction kinetics. The ERT system also highlights the very different conductivity profiles during precipitation when the stirred vessel is operated without agitation. This application is believed to be the first example of ERT applied to a precipitation process. ERT offers new possibilities for control action linked to spatio-temporal distributions of phases and components for industrial applications. (C) 2005 American Institute of Chemical Engineers.