Syngas adsorptions at 1.2 MPa and room temperature on a precipitated cobalt catalyst reduced by H, and on that used for high-pressure FT synthesis were studied by diffuse reflectance FTIR spectroscopy. The syngas adsorption on the freshly reduced sample gives rise to two weak bands at 2013 and 2033 cm(-1), which are assigned to linear adsorption of CO on metallic cobalt particles. The bands at 2013 and 2033 cm(-1) sharply increase by heating the sample after performing syngas adsorption for over 1 h, up to 100 degreesC in the high-pressure syngas flow. However, the intensities of the bands begin to reduce once the temperature remained constant. The increase of the bands during the heating may be attributed to recombination of the dissociated CO on the catalyst. High ability of the sample for CO dissociation indicates that fine metallic cobalt particles have been formed on the surface. Syngas adsorption on the sample after use for FT synthesis gives rise to two strong bands at 1999 and 2021 cm(-1), and several weak bands at 2150, 2025, 1897 and 1822 cm(-1). These bands do not increase while heating the sample to 100 C in the high-pressure syngas flow, indicating low ability of the sample for CO dissociation; so large particles have been formed on the surface. The bands at 1999, 2021 and 1897 cm(-1) are assigned to CO adsorption on cobalt carbides; the bands at 2025 and 1822 cm(-1) may be associated with sintered metallic cobalt species and the band at 2150 cm(-1) due to Co2+ species. The features of the high-pressure syngas adsorption indicate that, during FT synthesis, fine metallic cobalt particles on the surface are gradually transformed to a mixture of cobalt carbides, sintered metallic cobalt and small amount of cobalt oxides. (C) 2002 Elsevier Science B.V. All rights reserved.