The adsorption properties of the precipitated Co and CoMn catalysts with different ratios of cobalt to manganese reduced by Hz were studied by in situ diffuse reflectance FTIR using CO and CO + H-2 (syngas) as probe molecules. The adsorption measurements were performed by flowing the probe gases over the samples at a high pressure of 1.2 MPa. It has been found that metallic cobalt particles form on both Co and CoMn samples after reduction. This is manifested by the occurrence of the bands corresponding to the probes interacting with the metallic cobalt species. On the reduced Co sample itself, however, the molecularly adsorbed CO species are rather difficult to detect upon CO and CO + H-2 adsorption because of the dissociation of the adsorbed probes on the fine active metallic cobalt particles formed. On the manganese-incorporated samples, distinct bands appear in the region of 2100 and 1750 cm(-1) and their intensities increase with increasing the loading of manganese in the samples. They correspond to the species of CO Linearly adsorbed, bridge-adsorbed, and multiply bridge-adsorbed on Co-0 sites. The appearance of these adsorption species, especially the species of multiply bridge-bonded CO, indicates that the size of the formed metallic cobalt particles is larger than that on the reduced Co sample. Because of the moderate adjustment in size of the formed metallic particles by the incorporated manganese, the stability of the adsorption species are remarkably enhanced, as revealed by the spectra recorded from the thermal desorption of the adsorbed species. The size effect of the metallic cobalt particles formed may also be reflected by the improved properties of the catalysts to which manganese was added, such as reactivity, deactivation on stream and sulfur-tolerance in Fischer-Tropsch reaction.