Nanopowders of the magnetite–maghemite series have been synthesized by coprecipitation from aqueous solutions and by the sol–gel method, and a comparative comprehensive study of their structure has been carried out using X-ray diffraction analysis, scanning electron microscopy, low temperature nitrogen adsorption, and small-angle polarized neutron scattering. It has been established that the resulting iron oxide nanopowders are porous systems that, depending on the synthesis method, have a one-level, two-level (for powders obtained by aqueous synthesis), or three-level (for powders obtained by the sol–gel method) hierarchical structure organization with different scales and different types of aggregation for each of the structural levels, and the characteristic size for the larger level in both cases is > 45 nm. It has been revealed that the magnetic structure of the obtained iron oxide powders, regardless of the synthesis method, consists of superparamagnetic particles with a characteristic radius of magnetic R_m ~ 4 nm and magnetic–nuclear cross-correlations R_mn ~ 3 nm for powders obtained by the sol–gel method, and R_m ~ 5 – 11 nm and R_mn ~ 4 – 8 nm for powders obtained via aqueous route, depending on the synthesis conditions.