The interaction of small gold crystal tips with oxygen gas and CO/O-2 gas mixtures was studied by means of field ion microscopy (FIM). High-resolution FIM-images of clean tips were obtained with hydrogen and neon as imaging gas. At temperatures between 300 and 450 K the exposure of a clean An sample to O-2 gas at 100-1000 mbar, in the absence of an electric field, led to oxygen chemisorption and formation of a "surface oxide". The presence of an electric field of 12-15 V/nm was found to enhance the oxidation process. Exposure to CO gas at 300 K led to the removal of the surface oxide. This was associated with the occurrence of a wave front which started in the apex centre and extended to the outskirts of the tip sample. The build-up of the surface oxide and its titration by carbon monoxide was completely reversible. Our results strongly suggest that pure gold crystals are active catalysts for the CO oxidation at 300 K.