Adsorption of carbon monoxide on a nanosized apex facet of a [111]-oriented Pt field emitter tip was studied under high electrostatic field conditions (15-20V/nm) by field ion appearance energy spectroscopy (FIAES). Weakly bound highly mobile CO species have been detected and their possible role in surface reactions such as CO oxidation on Pt was elucidated. The FIAES results are compared with those of vibrational sum frequency generation (SFG) spectroscopy of CO adsorption on a macroscopic Pt(111) single crystal, in a gas pressure range from UHV to atmospheric. SFG and FIAES are complementary in that SFG mainly provides information on strongly bound CO molecules, whereas FIAES nearly exclusively detects weakly bound species. Both high-field and high-pressure studies indicate the presence of a compressed CO adlayer including weakly bound species. The high-field mimics the high gas pressure, leading to similar high adsorbate coverage, which is required for the formation of weakly bound highly mobile CO species, which may be the active entities in CO oxidation.