In this paper, methods of calculation of electronic polarizability of ions in anodic oxide films on aluminium, niobium and tantalum are discussed. It is shown that if considerable covalency of the chemical bond is taken into account electronic polarizability of metal and oxygen ions differs noticeably from that of "free" ions, determined by Fajans and Joos, Pauling and others. The Lorentz-Lorenz type equation which was derived makes it possible to calculate the ionic polarizability of metal ion. The results obtained are in good agreement with those of other authors. In case of amorphous materials with mixed ionic-covalent chemical bond a new method of local field calculation is proposed.