Hydrogenation of CO, CO2 and their mixtures has been comparatively studied in this work on a representative cobalt-based catalyst under typical Fischer-Tropsch synthesis conditions (T = 220 degrees C, P = 20 bar, GHSV = 4800 cm(3)(STP)/h/g(cat), H-2/COx = 2.45-4.9 mol/mol). In addition, the interactions of the adopted catalyst with CO, CO2 and their mixtures have been studied by FT-AR spectroscopy. When used alone, both CO and CO2 are easily hydrogenated over the adopted catalyst, with CO2 showing a reactivity higher then CO. However the selectivity of the two processes is extremely different, with over 90% of the products represented by methane in the case of CO2 hydrogenation. No evidence has been found for the involvement of different surface species in CO and CO2 hydrogenation, suggesting that the observed reaction products originate from the same intermediate. It is speculated that the different reactivity of the mixtures CO/H-2 and CO2/H-2 is due to the different adsorption ability of CO and CO2, which strongly affects the H/C atomic ratio on the catalyst surface. The higher H/C ratio resulting upon CO2 hydrogenation inhibits the chain growth, hence favoring the methanation reaction. In the presence of CO, CO2 is hardly hydrogenated and behaves as an inert species: this has been ascribed to a competition between CO and CO2 for the adsorption on the catalyst active sites. (C) 2008 Elsevier B.V. All rights reserved.