Study of mixing at the molecular scale (micromixing) gives an indication of the best way to carry out important processes such as polymerization, crystallization and competing fast chemical reactions. Rotating packed bed (RPB), in which gravity is replaced by a centrifugal force, plays an important role on intensification of mixing and transfer. In this study, by means of the diazo coupling and iodide-iodate test reactions, the micromixing efficiency of RPB was characterized at various operational conditions (e.g. rotational speed, liquid flow rate and reagent concentration). With a special configuration RPB which can achieve sampling along the radial position, the importance of inlet region of packing in RPB was experimentally confirmed firstly. Compared with other mixing devices, RPB is superior in improving micromixing efficiency. (c) 2006 Elsevier B.V. All rights reserved.