[en] The techniques of high resolution electron energy loss spectroscopy (HREELS), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) have been used to investigate the thickening of the oxide and hydroxide layers on Ni(110) single crystals at room temperature. Sputter cleaned and annealed samples were exposed to oxygen at pressures between 0.1 to 100 Torr and times of 30 min. to 15 hrs. The XPS data indicate that low pressure oxygen exposures result in the formation of a high binding energy O ls species which is confined to the near surface region of the overlayer film. In comparison with a Ni(OH)₂ standard, the high binding energy oxygen species is assigned to the oxygen state for Ni(OH)₂. The oxidation of nickel is divided into three reaction regimes, namely chemisorption, island nucleation and growth to coalescence, and thickening of the coalesced oxide. It has been proposed that the presence of an adsorbed charged oxygen species observed at high oxygen pressures may induce thickening of the oxide according to the Mott theory of field assisted ionic transport. At high pressure oxygen exposures, thickening of the NiO layer does not occur. Any observed increase in the overlayer thickness is confined to growth of the Ni(OH)₂ layer located at the near surface region. The ratio of O₂/H₂O in the oxidizing environment does not affect the composition of the final overlayer film but only the exposure at which this film is formed. The HREELS data indicate that the hydroxide species are not in an adsorbed chemical state but are due to the formation of Ni(OH)₂ in agreement with the XPS data