In order to experimentally estimate the rate-determining steps and the kinetic parameters of interfacial reaction models, e.g. for the design of reactive-extraction equipment, the Nitsch cell and its variants are broadly used. In the present study, an optimized Nitsch cell was designed with the help of a kinetic model from the literature to improve the quality of the measurements for the case of the system Zn/D2EHPA, which is a standard test system in reactive extraction. The determinant of the Fisher information matrix was used as a measure of the expected information obtained from the experiments as a function of apparatus geometry. Design parameters like the organic and inorganic phase volumes and the interfacial area between them have been optimized by maximizing the information obtained in each experiment. The results lead to an ideal geometry for such an apparatus, and show that significant improvement compared to current common practice is possible. (c) 2007 Elsevier Ltd. All rights reserved.