Activation and deactivation of Co foils during hydrogenation of carbon dioxide have been studied in dependence on pretreatment of the catalyst and time on stream, temperature and composition of the educt gas. Modifications of the surface due to activation and deactivation have been investigated by means of a combination of various methods, including temperature programmed processes as well as Auger electron spectroscopy and electron microscopy. Both oxidation/reduction of the surface and incorporation of oxygen and carbon in the bulk lead to marked changes in the surface structure and a considerable increase in surface area. The catalytic activity of the oxidized surface concerning the hydrogenation of CO2 is small, but increases strongly with reduction. Deactivation is accompanied with structural changes and proved to be reversible. Chemical poisoning was only observed after addition of hydrogen sulfide to the educt gas.