Ultrathin nickel oxide films were grown on the Ag(001) surface by evaporation of Ni in an O-2 atmosphere. The growth mechanism, composition, and structure of the NiO layers formed in the early stages of deposition have been investigated by LEIS, XPS, XPD, and LEED. We found that at O-2 pressures in the 10(-7)-10(-1) mbar range and sample temperatures between room temperature and 373 K, a nickel oxide layer forms showing a (2 x 1) LEED pattern. This is a phase of monatomic thickness which gradually covers the substrate surface upon increasing the surface coverage. LEIS and XPS data show that before the (2 x 1) layer is completed NiO(001) islands start to nucleate. The substrate surface is completely covered by NiO(001) for coverages higher than 2 ML. When NiO is deposited with the sample held at 453 K, we found that in the early stages of deposition metallic Ni is present in the subsurface region of the substrate. Upon further NiO deposition the oxide becomes the prevalent species on the surface. Under these conditions, the (2 x 1) phase is not observed. We found that the (2 x 1) phase is not stable upon annealing at 473-600 K and transforms into NiO(001). LEIS and XPD results indicate the formation of NiO islands 2 ML thick upon annealing. The structure of the (2 x 1) phase has been investigated by LEED intensity analysis (tensor LEED method). The best agreement with the experimental data is obtained for a structural model similar to the (111) surface of NiO. The effects of surface temperature and O-2 pressure on nickel oxide growth are discussed in terms of the low sticking probability of O-2 on Ag(001) and of the interaction between nickel and silver.