The reactive mixing process in a stirred tank has drawn much attention due to the complex interplay between the hydrodynamics and the chemical kinetics. However, there is still a lack of effective measurement techniques to explore the detailed information. To quantify the reactive mixing process, a novel reactive planar laser-induced fluorescence (reactive-PLIF) technique was developed to visualize how the two liquids mixed and reacted with each other. The main principle was to capture the process characterized by the fluorescence signal of the tracer dye (i.e., Rhodamine B), which varied in time and space because of being oxidized by the oxidant (i.e., the hydroxylradical (OH)-O-center dot) generated between Fe2+ and H2O2 (i.e., a Fenton reaction). The behaviors were recorded by a high-speed digital camera and quantitatively analyzed. The influences of the impeller rotation speed, the impeller position and the liquid properties on the processes were evaluated. The relationship between the reactive mixing and the physical mixing can be determined by the results from the (reactive-)PLIF measurements. This novel technique enabled the convenient measurement of liquid mixing process with reactions at a low cost. (C) 2010 Elsevier Ltd. All rights reserved.