Very small nickel oxide nanoparticles were prepared by a sol–gel procedure using nickel nitrate hexahydrate and ammonium hydroxide as precursors. The particles are in the range of 5 nm–11 nm. The x-ray diffraction (XRD) crys-tallography and high resolution transmission electron microscopy (HRTEM) were employed to characterize the samples. They were found to be polycrystalline in nature and fcc (NaCl-type) in structure, with the lattice parameter varying with annealing temperature. HRTEM pictures show that the as-prepared samples are hexagonal in shape. Positron annihilation spectroscopy was used to investigate the Doppler-broadened spectra of the samples. The S and W parameters revealed that the chemical surroundings and momentum distribution of the vacancy clusters vary with crystallite size.