Extensive new tomographic results are reported for barium sulphate precipitation in a stirred vessel under semi-batch operation exhibiting imperfect fluid mixing. An ITS System-2000 was utilized and a linear-back-projection algorithm employed for the creation of 3-D process images. It can be concluded that electrical resistance tomography (ERT) successfully captured the sequential electrical conductivity field behaviour with both sufficient spatial and temporal resolution. It also visualized the changes in the electrical conductivity field within the vessel when changing reactant feed locations. In addition, the mixing effects on the final crystal size distributions (CSD) were also experimentally investigated. Furthermore, it is confirmed that the data obtained by ERT is effective to validate a detailed precipitation process model. A new precipitation model has been developed and combined and hybridized with a networks-of-zones model which can represent imperfect mixing in a stirred vessel. The multi-zone mixing model with order 10(3) zones is commensurate with the vessel sub-division used for tomographic imaging. The images predicted by the 3-D hybridised model are compared with the tomographic measured results, including both temporal and spatial distributions, and they show good agreement.