The IR-visible sum-frequency generation (IVSFG) method was used to perform time-resolved vibrational measurements of CO/Pt(1 1 1). Irradiation of intense picosecond laser pulses (35 ps, 1064 nm) resulted in a drop in intensity, redshift, and broadening of the on-top CO stretching peak, followed by recovery to the original peak shape on the order of hundreds of picoseconds. The transient behavior of the CO stretching signal was found to depend on the CO coverage. The transient spectra at saturation coverage consisted of a single peak, whereas at low coverage (0.25 ML) the line shape became asymmetric and each transient peak could be deconvoluted to the multiple components. The origin of the remarkable change in the transient spectral features at the 0.25 ML surface is discussed. (C) 2003 Elsevier B.V. All rights reserved.