Infrared spectra of adsorbates and steady state reaction rates taken in situ during the reduction of NO and coadsorption of CO over a Pt(100) surface are presented. For surface temperatures ranging from 420 to 470 K and an NO pressure of 7.80 x 10(-5) Torr, IR spectra showed an absorption band at 1640-1656 cm(-1) that is attributed to adsorbed molecular NO. The mass spectral data revealed that N-2 gas is formed. When CO was admitted into the chamber, steady states corresponding to high and low rates of CO2 production were observed. The high reaction rate was accompanied by the appearance of a vibrational band at 1630 cm(-1). This band continued to grow until a critical CO pressure was reached. At this point, the 1630 cm(-1) band disappeared while absorption bands that correspond to a growth of CO adlayer began to appear. Except for the absence of an IR band that can be associated with molecularly adsorbed NO, the interaction of NO with CO at 500 K showed characteristics similar to those obtained at lower temperatures. Exactly the same behavior was observed during the reaction of CO with O-2 on Pt(100). The growth of the similar to 1630 cm(-1) band correlates with the high rate of CO2 formation, and its disappearance signals the transition to low reaction rate. These results strongly suggest that the formation of CO2 proceeds through an intermediate species that exhibits a vibrational band at similar to 1630 cm(-1).