The chemisorption and dissociation of CO on Ru(11 (2) over bar1) were investigated by using high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy. Three different adsorption states of CO can be distinguished. The most strongly bound beta-state, characterized by a C-O stretch frequency of 166 meV, is attributed to CO adsorbed in a fourfold hollow site of the Ru(11 (2) over bar1) unit cell. This state occurs only at low total coverage and dissociates at T>300 K. A more weakly bound state is alpha(1)-CO with a stretch frequency of 240-255 meV, attributed to CO on-top bonded to first and second layer Ru atoms. This species converts to beta-CO at moderate total coverage and T>360 K, increasing the amount of dissociated CO. The alpha(1)-CO species dominates the coverage regime up to 1.5 ML. The alpha(2)-CO species is most weakly bound in the coverage range up to 2 ML and is characterized by a stretch frequency of 220 meV. It is proposed to be located in twofold bridge sites. The ratio of on-top to bridge bonded CO is equal to three at saturation. The finite existence range for beta-CO is rationalized by a bond competition effect, due to neighboring alpha(1)-CO species destabilizing the beta-state at increasing coverage. Consequently beta-CO converts to alpha(1)-CO under these conditions. A decrease of the beta-state coverage via dissociation of CO may initiate the reverse process of alpha(1)- to beta-CO conversion. (C) 2003 American Institute of Physics.