We study the influence of the intensity of constant external magnetic fields on the structure, thermal properties, and electric conductivity of composites based on ED-20 epoxy resin and one of the metal oxides (Al2 O3 or Fe2 O3). It is shown that, under the action of the magnetic field, the Fe2O3 crystallites in the structure of the composite become finer (L ≈ 18 nm → 5.4 nm) and the densities of the initial epoxy polymer, the composites based on this polymer, and one of the metal oxides (Al2 O3 or Fe2 O3) increase, which, in turn, affects the other properties of these compounds, in particular, their specific heat and conductivity.