The structural, microstructural and morphological, as well as electric and dielectric, properties of nickel–zinc ferrite (Ni0.3Zn0.7Fe2O4) derived from sol–gel auto-combustion have been studied after sintering from 900 to 1300 C. The effect of heating rate has not been previously investigated and is reported here. X-ray diffraction showed a pure cubic spinel after calcination. Atomic force microscopy revealed nanosized particles after calcination, but scanning electron microscopy showed nanosized grains after sintering at 900 C. The heating rate has a marked effect on oxidation of Fe3+ to Fe2+, showing an additional approach to control charge carrier concentration in Ni–Zn ferrites (powder and monoliths). The heating rate also influences the average particle size and distribution. Grain size and resistivity of sintered pellets do not show significant change with heating rate, proving that resistivity is mainly dictated by the number of grain boundaries. The dielectric loss tangent curves at room temperature exhibit dielectric relaxation peaks attributed to the similarity in frequency of charge hopping between the localized charge states and external fields. The relaxation peak shifts to higher frequencies for ferrites with nanosized grains.