The activity and the selectivity of cobalt catalysts supported on a crystallised and on an amorphous zirconia were compared with cobalt supported on a gamma-alumina catalyst. The catalysts supported on zirconium dioxide were found to present a better reducibility of the active phase and also to be capable of hydrogen adsorption via a spillover mechanism. It is proposed that these properties could account for a better catalytic activity and an increase of the chain growth probability (alpha). At the same time, the estimated quantity of crystallised Co3O4 obtained after airflow calcination (for the same total cobalt loading) is related with the surface area of the support. (C) 2004 Elsevier B.V. All rights reserved.