Quantum-chemical calculations of the interaction of the molybdophosphoric Keggin heteropolyanion with the iron counter-ion are performed to explain the influence of the latter on the reducibility of iron-doped acid. It is shown that the electron transfer heteropolyanion-->iron counter-ion becomes possible only under hydration of the solid. The microscopic mechanism of this transfer is due to the modification of the relative position of the counter ion which enters into strong interaction with terminal oxygens of the heteropolyanion.