A technique using the visualization of reactive tracer mixing in 3-D is proposed for the quantification of mixing with chemical reaction in typical batch stirred vessel reactors. Images can be conveniently and inexpensively acquired using acid-alkali pulse tracer tests with suitable indicators. A demonstration is reported for the caustic soda-hydrochloric acid-phenolphthalein system, which produces dilute brine solution for safe disposal, on a 2.3 m(3) plant-scale stirred vessel. The fluid mixing and fast acid-alkali reaction have been modelled using simplified computational fluid dynamics (CFD) based on the network-of-zones model for O(10(5)) zones. The model predictions have been image reconstructed using AVS graphics. Using flow and mixing parameters independently acquired from inert tracer mixing tests, the close correspondence between experimental and theoretical images confirms that the micromixing must be close to perfect. The approach adopted quantifies both the local zone macromixing and macro-segregation effects for reactions faster than the fluid mixing rates and is useful for analysis of the shape and size of reagent-rich regions in semi-batch operation. Electrical resistance tomography (ERT) can suitably image the tracer mixing with reaction in 3-D according to the accompanying conductivity changes.