The newly developed tomographical dual wavelength photometry enables the measurement of the local intensity of segregation at a multitude of points inside the stirred vessel. This is done by injecting a mixture of an inert and a reacting dye into the vessel. The inert dye serves as a tracer for the macromixing, whereas the vanishing of the reacting dye shows the micromixing. The concentration fields of the dyes are measured simultaneously by transiluminating the vessel from three directions with superimposed laser beams of different wavelengths. The light absorption by the dyes is measured with CCD-cameras and these projections are used for the tomographic reconstruction of the concentration fields. Low Reynolds number measurements were performed with a combination of two Rushton turbines and a combination of two pitched blade impellers. The combination of the pitched blade impellers yields a good axial transport but a slow micromixing. The injection in the middle between the combination of the two Rushton turbines yields a faster micromixing, but the macrotransport is limited to the region between the stirrers. (C)2000 Elsevier Science S.A. All rights reserved.