The NMR relaxation method has been used for determing coordination numbers of oxygen-containing polyatomic ions in diamagnetic aqueous solutions. The results, obtained for 7 ions, make it possible to define the rules which govern the formation of ion hydration shells. Probably the developed model can be used for the estimation of results of quantum-chemical calculations and computer simulations. As an example, the molecular dynamics simulation of the sulfate dianion hydration shell has been analyzed.