In situ spectroscopy is the only approach to obtain reliable information on mechanisms and the role of intermediates in chemical reactions as well as on structure-reactivity relationships in catalysis . Especially, in heterogeneous catalysis, the application of a variety of in situ methods has gained a lively development during the recent two decades. But also in homogeneous catalysis, as well as in catalytic multiphase systems, in situ characterization methods are increasingly applied. A survey of techniques, which are most commonly applied in gas/solid and multiphase systems and the method-specific information, is presented in Table 3.1.The vibrational spectroscopies, Raman and Fourier transform infrared spectroscopy (FTIR) provide complementary information. While FTIR spectroscopy requires groups with a permanent dipole moment, Raman spectroscopy needs groups that have polarizable bonds. As a result, certain vibrations of functional groups are active or inactive in the infrared or Raman spectra, respectively. Thus, the spectra of molecules such as water or carbon monoxide show strong bands in the infrared spectra but only very weak bands in the Raman spectra. However, this is advantageous for measurements in aqueous systems. Furthermore, bands resulting from metal-nonmetal vibrations can be observed well with Raman spectroscopy. A disadvantage of Raman spectroscopy is the possible damage or fluorescence of the sample when irradiated with Raman laser light. In particular, the analysis of condensed aromatic systems can be perturbed by fluorescence. Consequently, the combination of both methods provides comprehensive information about the vibrational state of molecules or adsorbed species.Ultraviolet-visible (UV-vis) spectroscopy gives less distinct information about the molecular structure than infrared and Raman spectroscopies. Typically, the bands appearing in UV-vis spectra are rather broad. This is because of the simultaneous excitation of rotational, vibrational, and electronic transitions. The bands appearing in the UV-vis spectra of organic molecules are prominent for chromophores. These chromophores can be small parts of the sample molecule, but in extreme cases also the complete electronic shell of the sample molecule.