Charge distribution, one of the decisive factors influencing the properties of HTS materials, was studied by the XPS measurement of the core electron bond energies of the ions forming YBa 2Cu 3O 7- x. It was evidenced that oxygen hole monomers O -, eventually dimers O 22-, are present in the superconducting samples. The former behave as itinerant in the Cu(1)O(4) chains or can be fastened on the apical oxygens O(1). Formation of peroxidic anions O 22- requires, besides the thermodynamic condition, a suitable geometrical arrangement which is achieved in twin boundaries. Below Tc an addiitonal charge redistribution formally expressed as Cu 2++O 2-→Cu ++O - leads to the formation of an excess of holes which are probably transferred into Cu(2)O 22- planes.