The preparation and characterization of hcp and fcc Ni and Ni/NiO nanoparticles is reported. Ni and Ni/NiO nanoparticles were obtained starting from a precursor material prepared using a citric assisted Pechini-type method and, then, followed by a calcination of the precursor in air at either 400 or 600C for different times. The precursor was analyzed using thermogravimetric and differential thermal methods (TGA-DTA), and the resulting nanoparticles were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and vibrational sample magnetometry. Nanoparticles showed a phase transformation for Ni from hcp to fcc and/or to fcc NiO structure as the calcination time increased. The influence of the phase transition and the formation of NiO on the magnetic properties of the samples are discussed. © 2011 L. A. Garca-Cerda et al.