A series of Co doped Mn-Zn ferrites compounds with the formula Mn0.5Zn0.5 - COxFe2O4 (x = 0, 0.15, 0.25, 0.35 and 0.50) were successfully synthesized by polyethylene glycol-assisted coprecipitation and hydrothermal methods. The structural-characterization of the samples-was done using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR). All the samples found to have cubic spine) structure. The average crystallite size of all nanoparticles were estimated using Scherrer's formula and found to lie between 10 and 25 +/- 3 nm with small size distribution of particles prepared by hydrothermal method. The FTIR spectrum showed two absorption bands of tetrahedral and octahedral metal-oxygen sites. DC electrical resistivity varied from 4.12 x 10(7) to 8.32 x 10(10) ohm cm with cobalt doping. The dielectric measurements were performed from 20 Hz to 3 MHz and from 1 MHz to 1 GHz frequency ranges. The value of dielectric constant (epsilon') varies from 15.54 to 106.25 (1 MHz) and 6.73-16.48 (1 GHz) for all the samples at room temperature. Impedance spectroscopy was carried out from 20-Hz to 3 MHz, at room temperature to study the grains and grain boundaries effect. (C) 2013 Elsevier Ltd. All rights reserved.