The adsorption of CO on Ni(111), ion bombarded with various fluences of 1.0 keV Ar+ ions, has been investigated using photoemission from adsorbed xenon (PAX). After ion bombardment of the Ni(111) surface, various amounts of CO were adsorbed, followed by adsorption of xenon at 85 K, Two pressures of xenon were used in examining the 3d5/2 peak of xenon: 5 x 10(-6) and 7 X 10(-10) Torr. PAX data taken at both pressures show that CO selectively adsorbs onto the defect (step) sites created by ion bombardment. In addition, it was found that the amount of CO which could occupy a defect site previously occupied by one Xe atom varied from 10 to 2.5, depending on the ion fluence.