Modeling of liquid-liquid extraction processes involves the concentration of the extracted component directly at the interface. Currently, only very few and specialized methods are available for the direct measurement of these concentrations. Therefore a new, fluorescence-based measurement system with a high spatial resolution and a broad application spectrum was developed and tested. The detection principle is based on the use of fluorescent dyes excited by an argon ion laser. The intensity of the emitted light is dependent on the concentration of the extracted component in the very near surroundings of the dye. This intensity distribution is reproduced by an optical, microscope-based system onto a highly sensitive camera with a spatial resolution of I pm. This distribution is converted into a concentration profile at the interface using a calibration function and digital image processing routines. Measurements were performed in a commonly used stirred two-phase reactor modified to meet the requirements of an optical measurement system. It was shown that the concentration profiles at moving and nonmoving interfaces could be visualized with a resolution of I pm. The profiles formed at the interface differ significantly according to the kinetic of the used extraction system and the flow profiles in the reactor and can be used for further modeling of the extraction processes. (C) 2003 Elsevier Inc. All rights reserved.