The electronic interactions in Ag/Pt(111) and Cu/Pt(111) have been examined using X-ray photoelectron spectroscopy and ab initio MO-SCF calculations on cluster models. Ag and Cu atoms bonded to Pt(111) exhibit core-level binding energies that are smaller than those of bulk Ag or Cu. The negative core-level shifts correlate with an increase in the ability of the noble metals to adsorb CO. These electronic and chemical perturbations reflect the effects of a Pt-induced increase in the Cu 3d and Ag 4d electron populations. The behavior seen for Cu and Ag on Pt(111) is opposite to that found on early transition metals, where there is a decrease in the d electron populations of Cu and Ag with positive core-level shifts for the noble metals.