Tunable diode laser spectroscopy offers unique advantages for high resolution infrared reflection absorption spectroscopy (IRRAS) studies of monolayers on single crystal surfaces. Its greatest potentials lie in analyses of lineshapes less than 1 cm (FWHM), for resolving narrow features on absorption peaks, and for in situ solid-liquid interfacial studies, where high intensity is often required. Other IRRAS techniques introduce instrumental broadening sufficient to create considerable uncertainty in the intrinsic linewidth of very narrow peaks, and can mask sharp features. Using isolated or single mode output from a diode laser source provides resolution of iO3 cm, and thereby eliminates the need for instrumental deconvolution. However, the signal-to-noise (SIN) ratio is generally lower for spectra obtained with diode sources than with Fr-JR techniques, so that the most important applications using diode lasers have been studies of strong scatterers, such as CO on Pt(1 1 1). For this study, linewidths were measured for the C-O stretch mode for the ontop site of CO adsorbed at saturation coverage on a Pt(1 1 1) single crystal using diode laser sources. The measured linewidth at 100 K, using isolated modes, was 2.3 0.3 cm. The linewidth at 0 K was estimated to be 2.0 0.4 cm from variation of linewidths with temperature. Using the uncertainty principle, the 0 K linewidth corresponds to a lifetime of 2.7 0.6 ps, in reasonable agreement with recent lifetime measurements using time-resolved techniques.