The effects of sulfur poisoning on the chemisorption and on the methanation of carbon monoxide over Ru/TiO2 catalyst were investigated by FTIR spectroscopy and volumetric gas adsorption measurements. The CS2 molecules are eta’ bonded to Ru sites through one of the sulfur atoms and decompose to the constituent elements on thermal activation. Each S atom may deactivate 3 to 10 metal sites even at low coverages, the effect being more pronounced on the chemisorption of hydrogen. The deposited sulfur (and possibly carbon also) sterically hinders the formation of certain multicarbonyl and monocarbonyl species (nu CO in 2055-2140 cm(-1) region), which otherwise transform to methane via surface methylene groups in the presence of chemisorbed hydrogen and are found to play an important role in the low-temperature methanation activity of the studied catalyst. The Ru-CO species giving rise to lower-frequency vibrational bands are affected to a lesser extent. The presence of sulfur also results in the development of some new CO binding states which are weak and are identified with the CO and S coadsorbed at Ru sites of different oxidation states or of varying crystallographic nature. The CO adsorbed in these states is not reactive to hydrogen.