The main results of measurements of neutron magnetic form factors for rare earth systems with strong electronic correlations are considered. It is shown that unique information about the valence instable state in the f-electron system may be obtained by the use of neutron spectroscopy methods. These data allow to develop the adequate physical representations for valence fluctuating ground state. Examples of studies of the magnetic form factor for systems with intermediate valence based on Sm (SmB₆, SmS) and Eu (EuCu₂Si₂ type) are presented. Their significance for the formulation of realistic ideas about the physical nature of the intermediate valence state is discussed in detail.