The azo coupling of 2-naphthol and cresol novolaks was performed as an example for organic reactions in a microsegmented flow system with microspectrophotometric on-line characterization. A lithographically prepared glass/glass chip microreactor was used for the generation of segments and the mixing of reaction partners. The formation of segments and their transport behavior is strongly influenced by the addition of surfactants. The reaction rates were high in alkaline media at room temperature, so that the formation of dyes could be observed by a microscope-video system. The single reaction volumes were in the range between about 0.05 and 1 mu L. ne spectrum of each segment was registered with a measuring frequency of 45-50 Hz. The residence time of segments inside the optical measurement channels varied between 0.1 and 1.5 s and is in the range of 0.3 s for typical segment sizes of about 150 nL. The single reaction segments containing azo dyes as well as polymer-based azo dyes were stored in segment sequences inside capillary Teflon tubes.