Silica supported ruthenium catalysts have been prepared by incipient wetness impregnation with RuCl3 (aq). After drying and reduction these catalysts were additionally treated with carbon monoxide or CO/H-2 at 250-degrees-C. Due to this treatment the catalyst’s carbon monoxide chemisorption capacity significantly increased compared to normal reduction. At the same time the intrinsic reactivity for carbon monoxide dissociation is enhanced. These effects are attributed to the structural changes of the catalysts. A model, based on TPR, XPS, IR and TEM measurements, is proposed. The carbon monoxide treatment results in a catalyst that is better reducible and has larger metal ensembles.