Laser induced fluorescence technique (LIF) enables the measurement of the progress in mixing inside the mixing vessel. This is done by injecting a mixture of an inert and a reacting fluorescent dye into the vessel. The inert dye serves as a tracer for the macromixing. The reacting dye is changing its fluorescent characteristics while undergoing a fast chemical reaction with the vessel content and therefore shows the micromixing indirectly. The concentration fields of the dyes are measured simultaneously in an arbitrary plane using the two-colour LIF-technique. Areas of micromixing are detected by calculating the local degree of deviation from the concentration fields. Low Reynolds number measurements with a Rushton turbine show better macro- and micromixing for a dye injection closer to the stirrer shaft compared to a position closer to the centre of the main vortex. (C) 2004 Elsevier Ltd. All rights reserved.