CdS and Co-doped CdS nanoparticles have been prepared using a chemical precipitation method. X-ray diffraction analysis reveals that CdS and Co doped CdS nanoparticles exhibit hexagonal structure and the average grain size is found to lie in the range of be 2.2 to 3.8 nm. The Raman spectra of the Co-doped CdS nanoparticles exhibits a red-shift compared to that of CdS and this may be attributed to optical phonon confinement. The variation of impedance (Z' and Z"), conductivity (sigma), electric modulus (M' and M"), dielectric constant (epsilon') and loss tangent (tan delta) of CdS and Co-doped CdS nanoparticles as a function of cobalt concentration and temperature has been studied. (C) 2011 Elsevier B.V. All rights reserved.