Infrared spectroscopy is a significant technique of analysis, based on the characteristic absorptions of infrared radiation by molecules. This leads, especially in the mid-infrared between 400and 4000cm(-1), to strong distinctive absorption bands which allow the identification of molecules and molecular groups. The large strength of absorption demands an extensive sample preparation for measurements in transmission. Sample layer thicknesses of some micrometers have to be realized. In the measurement geometry of attenuated total reflection (ATR), however, these efforts are not necessary. In this case it is sufficient to drop some liquid, e.g., onto an ATR crystal. ATR probes make it possible to use this simple technique also for online monitoring of chemical reactors.