Ambient-pressure X-ray photoelectron spectroscopy (XPS) was used to investigate the reactions of CO, H-2, and their mixtures on Co foils. We found that CO adsorbs molecularly on the clean Co surface and desorbs intact in vacuum with increasing rate until similar to 90 degrees C where all CO desorbs in seconds. In equilibrium with 100 mTorr gas, CO dissociates above 120 degrees C, leaving carbide species on the surface but no oxides, because CO efficiently reduces the oxides at temperatures similar to 100 degrees C lower than H-2. Water as impurities or produced by reaction of CO and H-2 efficiently oxidizes Co even at room temperature. Under 97:3 CO/H-2 mixture and with increasing temperatures, the Co surface becomes more oxidized and covered by hydroxyl groups until similar to 150 degrees C where surface starts to get reduced, accompanied by carbide accumulation indicative of CO dissociation. A similar trend was observed for 9:1 and 1:1 mixtures, but surface reduction begins at higher temperatures.