The synthesis of Zn0.3Ni0.7–xCoxFe2O4 ferrites, where x = 0 – 0.7, is presented along with the results of studying their structures and microwave properties. These results show that cobalt substitution for nickel increases the unit cell parameter of the crystal lattice. The magnetic permittivity spectra contains two resonance peaks: low-frequency (1.31-2.04 GHz) related to the motion of domain boundaries and high-frequency (4.78-12.05 GHz) corresponding to the natural ferromagnetic resonance. An increase in the cobalt concentration shifts both resonances to higher frequencies. The Curie temperature of ferrites monotonically decreases as the cobalt concentration increases.