Adsorption behavior of carbon monoxide (CO) on 4 monolayer (ML) thick fcc-Fe films grown on Cu(l 0 0) has been investigated with IR reflection absorption spectroscopy (IRRAS) and low energy electron diffraction (LEED). The as-grown film gives a 5 x I LEED pattern which remains unchanged during CO exposure at 90 K. Simultaneous IRRAS measurements show that the band due to CO adsorbed on the on-top site dominates the spectra at saturation coverage. In contrast CO exposure at room temperature leads to change in the LEED pattern from 5 x I to diffuse c (2 x 2) accompanied by preferential adsorption of CO on the bridge site of the film surface. These results suggest that room temperature CO exposure gives rise to a change in the reconstructed surface of the as-grown film and c(2 x 2) overlayer of CO adsorbed on the bridge site, while CO adsorption takes place on the on-top site of the 5 x 1 reconstructed surface with no long-range ordered overlayer at 90 K. (C) 2002 Elsevier Science B.V. All rights reserved.