We report on the impedance spectroscopic characterization of Sm and Ho doped Ni ferrite materials, namely NiO. Fe1.925 Sm0.075 O3 and NiO. Fe1.925 Ho0.075 O3, to demonstrate their improved electrical properties compared to pure NiO. Fe 2 O3. Sm and Ho doped Ni ferrites crystallize in the cubic inverse spinel phase with a very small amount of SmFe O3 and HoFe O3 as the additional phase, respectively. Atomic force microscopy measurements indicate that the bulk grains are approximately 2-5 μm in size while the grain boundaries are thin compared to bulk grains. Frequency variation of the dielectric constant shows the dispersion that can be modeled with a modified Debye's function, which considers the possibility of more than one ion, contributing to the relaxation. The resistivity values (at 3.5 KHz) of NiO. Fe2 O3, NiO. Fe1.925 Sm0.075 O 3, and NiO. Fe1.925 Ho0.075 O3 compounds are found to be 0.1× 104 m, 0.5× 10 4, m and 0.8× 104 m, respectively. Impedance spectroscopic analysis indicates the different relaxation mechanisms and their variation with temperature, bulk grain and grain-boundary contributions to the electrical conductivity (Rg), and capacitance (Cg) of these materials. While the conductivity in pure NiFe O4 is predominantly due to intrinsic bulk contribution (Rg =213 k and Cg =4.5× 10-8 F), NiO. Fe1.925 R 0.075 O3 (R=Sm,Ho) exhibits distinct grain and grain-boundary contributions to the conductivity. © 2011 The Electrochemical Society.