Nanocrystalline Co-Ni ferrites bearing chemical formula Co0.5Fe2-xNi0.5+1.0xO4 for x ranging from 0.0 to 0.4 with the step increment of 0.1 were successfully synthesized by sol gel auto-combustion method. The energy dispersive X-ray analysis (EDAX) results give relevant information for the homogenous mixing of the Co, Fe, and Ni atoms as expected from the synthesis. The phase identification of the materials by XRD reveals single phase with cubic symmetry. The presence of functional group was identified by Fourier transform infrared spectroscopic studies. The dielectric parameters such as dielectric constant ((epsilon'), dielectric loss (epsilon '') & dielectric loss tangent (tan delta) have been studied at room temperature in the frequency range 42 Hz-5MHz and is explained in the light of interfacial polarization, arising from the heterogeneous nature of ferrite structure. The decrease in DC resistivity with increasing Ni concentration is attributed to the Verwey mechanism between Fe2+ <-> Fe3+, Co2+ <-> Co3+ and Ni2+ <-> Ni3+. Ni-doped nanocrystalline cobalt ferrite samples exhibit a very large value for dielectric constant of the order of 10(13). Complex impedance analysis has been used to separate grain and grain boundary in the studied samples. It is observed that saturation magnetization (Ms) decreases with increase in nickel contents which is attributed to the substitution of magnetic Fe3+ ions of 5 mu(B) by less magnetic Ni2+ ions of 2 mu(B). The analysis of the Mossbauer spectra shows the hyperfine field, relative % area and isomer shift decreases whereas quadruple splitting and line width increases at A- and B-sites on increasing the substitution of Ni2+ ions. (C) 2012 Elsevier B.V. All rights reserved.