Dissociative chemistry of CO on an atomically stepped single crystal Ru(1 0 9) surface was studied using the combination of vibrational spectroscopic and thermal desorption measurements. CO molecules on stepped Ru are by desorption and dissociation together upon heating to 480 K. Contrary to the atomically smooth Ru(0 0 1) surface, the step defect sites on the Ru(1 0 9) surface facilitate CO dissociation. Adsorbed atomic O and C then recombinatively desorb near 520 K. This low-temperature mobile carbon species is likely to be an intermediate in the Fischer-Tropsch synthesis on Ru. (C) 2002 Elsevier Science B.V. All rights reserved.