Sum-frequency spectroscopy (SFS) was used in an attempt to detect the platinum-carbon vibration of CO adsorbed on Pt(111). The international free-electron laser FELIX at the FOM Institute, Rijnhuizen, provided the required tunable far-infrared (19-23 mu m) source, while complementary measurements in the C-O stretch region (4.7-5.1 mu m) were performed at the University of Oxford with a conventional nanosecond laser system. Ordered Pt(111) surfaces were prepared by the H-2/O-2 flame annealing approach and CO monolayers were produced by exposure of the Pt crystal to gaseous CO in a flow reactor. The monolayers were characterized by sum-frequency (SF) measurements of the nu(C-O) vibrational frequency. The CO adsorbed primarily in the terminal (atop) configuration, with a nu(C-O) frequency of around 2078 cm(-1). In the far-IR region, the non-resonant background from the Pt substrate could readily be detected by SFS, but there was no evidence for the nu(Pt-CO) mode. Direct laser-induced desorption and thermal desorption of CO are unlikely under the experimental conditions. It is therefore probable that the intrinsic cross-section of the Pt-CO mode is too low for easy detection by SFS. The implications for the use of SFS to detect metal-adsorbate vibrational modes are discussed in light of these findings.