Contact potential measurements for clean, single crystal surfaces of copper compared with a gold polycrystalline reference surface have shown that the values of (ø cu− ø au) for the (111), (100) and (110)Cu planes are +0.84±0.019, +0.455 ±0.054 and +0.22 ± 0.019 eV respectively. The interaction of oxygen with these planes, at room temperature, leads to a work function increase in all cases. These increases are 0.125, 0.39, 0.675 eV for the (111), (100) and (110) planes respectively. Oxygen adsorption at 77°K leads to values essentially similar to the room temperature values, except in the case of the (110) plane, where the work function increases by only 0.36 eV. Warming such a (110) surface to room temperature allows further oxygen adsorption and a final work function increase of 0.63 eV. There is evidence that oxygen is being incorporated in the (110) surface at room temperature. The behaviour of oxygen on these surfaces is reconciled by means of a “hole” theory. Adsorption of carbon monoxide on the (111), (100) and (110) planes leads to a work function increase of 0.01 eV, 0.11 eV and 0.02 eV respectively, with little difference between the low temperature and room, temperature values.