For the evaluation of mixing processes in microreactors, it is necessary to be able to investigate them in detail. For this purpose, a novel optical measuring system is presented, which allows the visualization of mixing and diffusion-limited processes as well as concentration differences in fluid films. It consists of focusing optics with a microscope objective. In combination with a Raman spectrometer, various processes such as diffusion in the microreactor can be measured and investigated non-invasively.