The coadsorption behavior of CO and hydrogen on Co(0001) during exposure to mixtures of H-2 and CO at pressures up to 300 mbar and temperatures between 298 and 490 K has been studied by in-situ polarization modulation reflection absorption infrared spectroscopy (PM-RAIRS). At 490 K it is shown that adsorbed CO molecules attached to cobalt atoms at step edges ("defect sites") disappear. We explain this observation in terms of hydrocarbon formation at defect sites, which blocks the adsorption of CO at these positions. To support this explanation, both ex-situ X-ray photoelectron spectroscopy measurements and kinetic measurements have been performed. The kinetic measurements provide direct evidence for a link between the number of defect sites at the surface and the total Fischer-Tropsch activity.