To establish the physicochemical nature of structure formation, a study has been made of high-viscosity crude oils and natural bitumens from a number of fields of Tatarstan and the Ul＇yanovsk region, which are noted for a wide range of viscosities (0.03-700 Pa sec). The established relationships between the rheological properties and compositional characteristics, obtained by means of molecular spectroscopy (IRS, the UV and visible region of the spectrum, EPR, high-resolution H-1 and C-13 NMR, and also pulse NMR to obtain the parameters of proton spin-lattice relaxation) combined with traditional methods for crude oil investigation, showed that the main mechanisms of structure formation in high-viscosity crude oils and natural bitumens, governing their high viscosities and the appearance of structural-mechanical strength, are intermolecular interactions of the high-molecular weight fragments of the asphaltene-resinous substances, connected with the strong properties of paramagnetism of the polyaromatic structures in their composition. It was found that a sharp qualitative change in the rheological properties of high-viscosity crude oils and natural bitumens, making it possible to carry out their nominal classification for a viscosity of 10 Pa sec, is observed when a critical concentration of asphaltene-resinous substances is achieved in them (similar to 35%) with a smooth change in the chemical composition. The common physicochemical nature of structure formation and the genetic similarity of the composition of high-viscosity crude oils and natural bitumens substantiate a common approach to selecting the directions of processing and combined development of fields of natural bitumens and lower-lying high-viscosity crude oils, which can ensure profitable management of resources of natural bitumens, which are considered as one of the main alternative sources of hydrocarbon, petrochemical and other feedstock.