A theoretical treatment of X-ray chemical shift (Ch.S.) was proposed taking into account different contributions to it. This theoretical approach was applied to analyse the effects following the formation of solid solutions of Cu²⁺ in host lattices of the highly symmetrical oxides NiO and MgO. The interpretation of the X-ray effects is based on the correlation observed between the Ch.S. and magnetic data and on the results of the present theoretical work. The most important result is the conclusion that the innermost Kα X-ray transitions of a magnetic atom follow a mostly parallel orientation of the spins of uncoupled valent electrons and the inner electron holes involved into X-ray transition.