Static mixing in chip devices can easily be monitored if a construction with transparent channel segments is used. Therefore, a mixer consisting of a three-layer arrangement glass/Si/glass was developed. All channel segments etched in the central Si chip can be visually observed or used for optical measurements in transmittance due to the completely transparent glass layers. The other channel segments are etched in both glass layers and are observable in reflection mode. The chip mixer contains eight split and recombine units. It was tested by the formation of a coloured product from two non-coloured aqueous solutions as well as by the interdiffusion of a fluorescent dye in glycerol/water mixtures. The microscopic measurements suggest that good mixing efficiency is achieved after all eight split and recombine units up to flow rates of 200 mul/min even in case of more viscous liquids. In case of lower flow rates (20 mul/min and less), sufficient mixing by interdiffusion already takes place in the first three conversion transducers. Additionally, the use of the reactor is demonstrated in a micro modular photochemical experimental arrangement. (C) 2004 Published by Elsevier B.V.