The authors studied the electric structure of two high-temperature superconductors, with particular emphasis on photoemission resonance effects and on collective excitation phenomena. The photoemission resonances simplified the identification of valence spectral features. In particular, they provided evidence for Cu satellite features, whose analysis is of crucial importance in the identification of the valence state of Cu. From the energy loss spectra, they derived the plasmon energies and the energies of quasi-optical transitions in an energy range up to 110 eV. These data were interpreted in terms of the initial states derived from photoemission and of possible empty states immediately above the Fermi level. The study included a careful search for changes in the photoemission and energy loss spectra during the phase transition of YBa₂Cu₃O_7-x. Based on previous data on the charge-density-wave-like phase transition in TiSe₂, the results set an upper limit for the width of the superconductivity gap, and are relevant to proposed superconductivity mechanisms such as incipient charge-density-wave and/or excitonic pairing