Nanocrystalline Co substituted Ni–Zn ferrites with the general formula Co_xNi_0.6−xZn_0.4Fe₂O₄ (x = 0.0 to 0.6) were prepared by a precursor combustion method. Average crystallite size, as estimated by using a Scherrer method and the particle size obtained from transmission electron microscopy (TEM) techniques, was found to be in the range of 10–30 nm. EDX and XRD analysis confirms the presence of Co, Ni, Zn, Fe and oxygen and the desired phases in the prepared nanoparticles. Selective area electron diffraction (SAED) analysis confirms the crystalline nature of the prepared nanoparticles. It is observed that Co substitution has a pronounced effect on the magnetic properties such as M_R, M_S and H_C and also on the Curie temperature (T_C), which is found to decrease from 420 °C for non-substituted Ni–Zn ferrites to 325 °C for the highest substitution of x = 0.6. These effects are assigned to the higher magnetocrystalline anisotropy of Co than Ni and to the size dependent existence of single domain–superparamagnetic particles distribution. Comparatively, the dominant existence of single domain particles with Co substitution over superparamagnetic particles in non-substituted nickel zinc ferrites is extensively investigated in this article.