The model explaining the phenomenon of polyvalent metal’s high mobility in complex compounds have been analyzed on the basis of principal role of covalency, high energy and easy polarizability of metal-oxygen bonds. The transfer of V, Nb, Mo and W may be considered as a crawling due to rearrangement of these bonds. The transfer is facilitated in the presence of pyramidal defect centres of the type [WO3Vo](0) or [NbO3Vo](-) and is promoted by their potential ability to shape-invertion and conformational changes. In polycrystals an important role belongs to MoO3-, WO3- and V2O5-based mobile adsorbed phases at the grain boundaries.