We report on the impedance spectroscopic characterization of Sm and Ho doped Ni ferrite materials, namely NiO center dot Fe1.925Sm0.075O3 and NiO center dot Fe1.925Ho0.075O3, to demonstrate their improved electrical properties compared to pure NiO center dot Fe2O3. Sm and Ho doped Ni ferrites crystallize in the cubic inverse spinel phase with a very small amount of SmFeO3 and HoFeO3 as the additional phase, respectively. Atomic force microscopy measurements indicate that the bulk grains are approximately 2-5 mu m 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 center dot Fe2O3, NiO center dot Fe1.925Sm0.075O3, and NiO center dot Fe1.925Ho0.075O3 compounds are found to be 0.1 x 10(4) Omega m, 0.5 x 10(4), Omega m and 0.8 x 10(4) Omega 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 (R-g), and capacitance (C-g) of these materials. While the conductivity in pure NiFeO4 is predominantly due to intrinsic bulk contribution (R-g = 213 k Omega and C-g = 4.5 x 10(-8) F), NiO center dot Fe1.925R0.075O3 (R = Sm, Ho) exhibits distinct grain and grain-boundary contributions to the conductivity. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3534800] All rights reserved.