Co/Al2O3 catalysts, prepared from four commercial aluminas, with different cobalt loading have been studied for the carbon monoxide hydrogenation (Fischer-Tropsch synthesis). The effects of reaction and reduction temperatures, deactivation, cobalt content and texture of supports on the catalytic properties have been especially examined. An increase in the reaction temperature improves the activity and the selectivity for light products. The product distribution follows the Schulz-Flory model. For the 15 wt.% Co on powder alumina catalyst, the activity and the paraffin/olefin ratio C-6(-)/C-6(=) follow a logarithmic decrease with the time-on-stream. Characterization by XRD, XPS and SSA measurements have shown the presence of heavy hydrocarbons (C-12(+)) in the tested catalyst porosity. The cobalt loading and the porosity of supports modify the catalytic properties through their effects on the reducibility of the active phase. An increase in the extent of reduction improves the activity and the selectivity for high molecular hydrocarbons weight. The evolution of the chain growth probability (a) is closely related to the degree of reduction. In the absence of secondary reactions, a seems to be limited at a maximum value.