Hyperthermal Ar atoms were scattered under grazing incidence (qi=60°) from a CO-saturated Ru(0001) surface held at 180 K. Collision-induced desorption involving the ejection of fast CO (~1 eV) occurs. The angularly-resolved in-plane CO desorption distribution has a peak along the surface normal. However, the angular distribution varies with the fractional coverage of the surface. As the total CO coverage decreases, the instantaneous desorption maximum shifts to larger outgoing angles. The results are consistent with a CO desorption process that involves lateral interaction with neighbouring molecules. Furthermore, the data indicate that the incident Ar cannot readily penetrate the saturated CO overlayer. Time-of-flight measurements of scattered Ar exhibit two components, one fast and one slow. The slow component is most evident when scattering from the fully-covered surface. The ratio and origin of the components vary with the CO coverage.