We present results of self-consistent tight-binding calculations of CO chemisorption at epitaxial Pd and Pt monolayers deposited at Re(0001) and Ru(0001) substrates, respectively. The most striking finding is that the CO adsorption in the atop position is affected by the substrate presence much less than at the 3-fold site. The reduction of the molecule-metal bond strength in the latter geometry is accompanied by a lowered charge sigma donation and especially pi back-donation. The additional core-level shifts on the CO metal neighbors derived within the initial-state scheme are positive and are Larger for the atop geometry.