CO adsorption on a Pd(111) single-crystal surface was investigated using in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) within the pressure range 10(-6)-800 mbar. The coverage-dependent CO overlayer structures found on the Pd(111) surface are identical throughout this pressure regime, that is, no new surface species at elevated pressures or adsorbate-induced substrate reconstructions were observed. The transition from an adsorbate superstructure dominated by bridged-bound CO to an adsorbate overlayer having 3-fold/atop CO sites was followed by varying the adsorbate pressure over 9 orders of magnitude. The derived phase diagram indicates an apparent activation energy of 44.35 +/- 1.63 kJ/mol for the bridged-to-3-fold-hollow/atop transition. A comparison between these data and recent sum frequency generation (SFG) results is made.