This thesis should help to extend the database of the Fourier-Transform-Infrared (FTIR) spectrometer at the Chair of Waste Processing Technology and Waste Management. The aim for the future is the qualitative and quantitative phase analyses of slag and ash samples. Therefore, common minerals in slags and ashes (anhydrite, calcite, chromite, hematite, magnetite, periclase, quartz, wuestite) were analysed. These minerals are processed and can be analysed directly with the attenuated total reflection (ATR) technique. Additionally, the KBr pellet method was used to obtain spectra using the transmission mode. All spectra are saved in a database. A number of different mixtures were used to conduct quantitative analyses with the software ResPro. This software offers many possibilities of editing and analysing the spectra. The possibilities of the software are discussed and an instruction on how to do a quantitative analysis is also given in this thesis. X-ray diffraction patterns were used to determine the phase composition of the slag and ash samples and compared with the FTIR-spectrometer data. It has been shown that FTIR-spectrometry is an excellent tool for qualitative but not yet for quantitative phase analysis. Other methods should be preferably used for quantitative measurements.