Formation of Co-0 phases with different surface structure over 10 wt% Co/Al(2)0(3) and their catalytic properties were induced by pretreatments in H-2 at 570 K for 1 h or 20 h. Electronic behaviour of the Co-0 phase, which consists of small (after 1 h reduction) or large bulk-like particles (after 20 h reduction), did not change during the CO hydrogenation after 5 h on stream as was determined by XPS. On the basis of the measured C-2+ hydrocarbon selectivities the CO molecules are suggested to dissociate on small Co particles to a larger extent than on large cobalt particles. The slight decrease in the catalytic activity with increasing time on stream obtained for the long-term reduced sample is explained by the change in the surface Co-0 content detected by XPS. The increase in the catalytic activity along with the change in olefin selectivity, measured for the sample reduced for 1 h, is interpreted by the change of a reaction path involving the Co-0-support interface during the initial period of the reaction.