The adsorption of CO on Co(0001) has been investigated in situ by polarization modulation infrared reflection absorption spectroscopy (PM-RAIRS), which has been applied for the first lime in a study of a model system for a heterogeneous catalyst. The CO/Co(0001) system was studied in the pressure range from 10(-10) to 600 mbar at temperatures between 300 and 550 K, showing the in situ potential of PM-RAIRS and the significant scope of this method for catalysis research. Linearly and bridge-bonded CO species could be distinguished on well-annealed surfaces. High-pressure RAIRS experiments done at room temperature were in agreement with previous low-energy electron diffraction (LEED) investigations in ultrahigh vacuum (UHV) at 100 K,(3,4) indicating a transition in the CO layer from a (root 3 x root 3)R30 degrees to a (2 root 3 x 2 root 3)R30 degrees structure with increasing CO coverage. By comparison of well-annealed and Ar-sputtered (defective) surfaces, we could identify, at a high frequency of around 2080 cm(-1), a CO species attached to defect sites. It is shown that annealing at 450-490 K at 100 mbar of CO pressure leads to the creation of defects at the cobalt surface. The defects influence the structure of the CO overlayer, The nature of this "defect"-bound CO is discussed. Postreaction X-ray photoelectron spectroscopy (XPS) showed the development of surface carbide upon annealing in CO, which is in good agreement with the vanishing of the RAIRS signal of adsorbed CO at temperatures above 520 K.