wCarbon monoxide (CO) adsorption at 90 K on Cu(100), (110), and (I 11) covered with submonolayer Fe at room temperature has been investigated using infrared reflection absorption spectroscopy (IRRAS). Saturated adsorption of CO on the 0.3-monolayer (ML)-thick Fe/Cu(I 11) bimetallic surface yields two C-O stretch bands at 2075 and 2015 cm(-1) due to adsorption on the uncovered Cu and on deposited Fe, respectively. The former band agrees in frequency with the C-O stretch band on the clean Cu(1 11) surface, indicating that the deposited Fe does not affect the Cu surface. In contrast, on the 0.3-ML Fe/Cu(100) surface, an additional C-0 stretch band emerges at 2104 cm(-1) on the high-frequency side of the band (2090 cm(-1)) due to CO adsorbed on the uncovered Cu surface. This band arises from CO bound to Cu atoms formed by intermixing of the deposited Fe and substrate Cu atoms. Even more intense Fe-Cu intermixing occurs on the 0.3-ML Fe/Cu(I 10) surface; for saturated adsorption, the corresponding band is observed at 2108 cm(-1), and its relative intensity to the C-O band on the uncovered Cu surface is much stronger than on the 0.3-ML Fe/Cu(100) surface. This severe intermixing at the Cu(110) substrate surface is explained in terms of a high surface energy and the specific surface lattice structure.