Infrared reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD) were used to investigate carbon monoxide (CO) adsorption on 0.25-1.0 monolayer (ML)-thick Fe deposited Pt(100) (Fe_{x(x=0.25-1.0ML)}/Pt(100)). The cleaned Pt(100) surface shows 5 × 20 reconstruction (Pt(100)-hex). The 1.0-L-CO exposure to the Pt(100)-hex at 300 K yields linearly bonded CO-Pt bands at 2089 and 2081 cm^-1. They are ascribable respectively to adsorbed CO on 1 × 1 and hex domains. Low-energy electron diffraction (LEED) patterns for the 343-K-deposited Fe_x/Pt(100) change from blurred hex (0.25 ML) to 1 × 1 (>0.5 ML), indicating that the outermost surface structures changed through Fe thickness. Regarding the Fe_0.25ML/Pt(100), two bands are located at 2082 and 2069 cm^-1 for the 1.0-L-CO exposed surface. The Fe_0.5ML/Pt(100), however, reveals a single absorption band at 2050 cm^-1. The CO exposure to the Fe_1.0ML/Pt(100) generates a broad and more red-shifted band at 2027 cm^-1. In contrast, the LEED pattern for Fe_0.5ML/Pt(100) fabricated at 873 K shows hex reconstruction. The IRRAS spectrum recorded after 1.0-L-CO exposure for the 873 K deposited Fe_0.5ML/Pt(100) shows an absorption band at 2073 cm^-1 with narrow bandwidth. The CO desorption temperature for the Pt(100)-hex (about 520 K) was shifted to a lower temperature by Fe deposition: the temperatures are about 470 and 430 K for the Fe_0.5ML/Pt(100) fabricated substrate temperatures of 343 and 873 K. Based on IRRAS and TPD results, CO adsorption behavior on the Fex/Pt(100) is discussed. [DOI: 10.1380/ejssnt.2009.245]