Highly crystalline CoxNi0.5−xZn0.5Fe2O4 (x=0.0, 0.2, 0.4) nanoparticles of about 5 nm in size were synthesized by the polyol method and subsequently sintered at 700 °C for 4 h. The structural, electrical, and magnetic properties of the sintered ferrites were investigated using various techniques including X-ray diffraction, thermal analysis, infrared and energy dispersive X-ray spectroscopies, transmission electron microscopy, and vibrating sample magnetometry. The lattice parameter increased linearly with Co content in agreement with increasing substitution of Ni2+ for Co2+. The sintered powders are composed of nanograins with average size ranging from ~30 to ~40 nm. The electrical study showed a resistivity decrease with increasing temperature indicating a semiconducting behavior. In addition, the dc conductivity was found to follow the Arrhenius law with a slope change observed at a critical temperature, the Curie temperature. The variation of the dielectric properties (dielectric constant and the dielectric loss) with frequency and temperature was explained on the basis of Maxwell–Wagner type of interfacial polarization. At room temperature, the best characteristics (high resistivity, high permittivity, and low loss factor) were found with the Co composition x=0.2. They were found to be considerably better than those of the unsubstituted Ni–Zn bulk ferrite. Magnetic investigation showed that all the sintered particles exhibit superparamagnetic behavior at room temperature. Additionally, the saturation magnetization and the Curie temperature showed similar trend; they increased slightly from x=0.0 to x=0.2 and then they decreased notably for the composition x=0.4. This can be attributed to the changes of superexchange interactions between cations in the spinel-type sublattices on Ni substitution.