The hydrogenation of CO over supported cobalt catalysts has been studied using in situ FTIR spectroscopy and gravimetry at P = 6 bar, T = 473 - 723 K and H2/CO = 2 - 3. On both silica-and alumina-supported catalysts IR absorption bands corresponding to linearly adsorbed CO on metallic cobalt were observed. On alumina an additional pair of bands at lower frequencies was attributed to bridge-bonded CO. Absorption bands corresponding to adsorbed hydrocarbons (3050 - 2700 cm-1) and to oxygen containing species (1800 - 1200 cm-1) were found to correspond to adsorbed products or unreactive species. The gravimetric studies showed a significant difference between the supports. On the silica-supported catalyst the weight uptake decreased with increasing temperature (473 - 573 K). The weight increase during reaction was attributed to adsorbed hydrocarbon reaction products. On the alumina-supported catalyst the weight uptake increased with increasing temperature, and there was also a significant weight increase with the support alone. Most of the weight uptake can be attributed to the formation of stable formate and carbonate species on the alumina support. At 723 K the deposits formed were stable in H2, and the shape of the curves indicated different mechanisms for deposition of material. In particular the Co/Al2O3 sample showed a very high and linear rate of weight gain, which was an order of magnitude higher than for the other samples.