Using density functional theory (DFT) in conjunction with ultraviolet (UPS) and X-ray photoelectron spectroscopy (XPS), we investigated a number of complexes and macromolecules. We have shown on a large set of UPS, XPS, and DFT data that the calculated Kohn–Sham energies of organic and metalorganic complexes can be used as approximate ionization energies (IEs). It is possible to evaluate IEs with an accuracy of 0.1 eV with the density functional approximation (DFA) defect approach. This method has been successfully tested on a large number of boron b-diketonates and d-metal chelate and sandwich complexes. We interpreted the bands in the valence region of the XP spectra of macromolecular organosilicon compounds in the solid state by taking into account the density of states and the ionization cross-sections. According to DFT calculation results, the one-electron states in the valence region of the model compounds correlate with the positions of the spectral band maxima.