The role of oxygen in determining the surface characteristics of nickel catalysts has been investigated by work function, photoelectric and chemical reactivity studies. The possible limitations of any one particular method are discussed. Surfaces which have been exposed to oxygen (10–4 mm min) at 23° exhibit the characteristics expected of a surface oxide although the chemical reactivity is not that associated with NiO. Oxide structures of low work function can form, and these were shown to have specific adsorption characteristics. The adsorption of carbon monoxide and oxygen and the catalytic oxidation of carbon monoxide by heavily oxidized nickel films were studied. Two distinct states of adsorbed oxygen were recognized. A substantial decrease in work function occurred on adsorbing carbon monoxide, the decrease being greater the higher the temperature. At 170° and 1.5 mm the decrease was 0.85 eV the major portion of this arising from a space charge term. The oxide work function remained virtually unchanged during catalysts at 23°. This was due to a low energy state of adsorbed oxygen inhibiting the adsorption of carbon monoxide thereby preventing the development of a spacecharge.