Copper ferrite nanoparticles with cobalt and nickel substitutions of different content were developed by a sol–gel autocombustion method. The synthesis conditions caused a structure of crystallite and morphology variation, resulting in their properties becoming markedly different for specific shielding applications. The synthesized ferrite nanoparticles are of several tens of nanometers in diameter with a tendency for their size to decrease with increasing cobalt or nickel ion content. Epoxy composites with Co-substituted copper ferrites are characterized by quiet less values of the real part of dielectric permittivity, than composites with Ni-substituted copper ferrites in the frequency range 26–40 GHz. The frequency dependences of the imaginary part of the dielectric permittivity of 20 wt. % Cu1-xCoxFe2O4 (x = 0.0; 0.2; 0.4; 0.6; 0.8)/epoxy composites increase slightly with frequency, and a significantly increasing frequency dependence is observed for 20 wt. % Cu1-xNixFe2O4 (x = 0.0; 0.2; 0.4; 0.6; 0.8)/epoxy series of samples, and with increasing level of substitution the effect is more pronounced.