Models for simulating the performance of extraction columns have been extended to include partially miscible solvents. Simultaneously, a comprehensive experimental study was carried out in which the concentration profiles of all components and all relevant hydrodynamic parameters in a pilot plant were measured, using liquid systems recommended by the European Federation of Chemical Engineers. True mass transfer coefficients were determined from the experimental data using a computer-aided curve-fitting technique to allow for the influence of backmixing. The coefficients obtained in this way were similar to those calculated for single drops.