A method of temperature-programmed reduction (TPR) combined with simultaneous measurement of magnetization was used to show that the porous structure of a support markedly affects the reduction kinetics of supported Co Fischer-Tropsch catalysts. It was shown that 30% Co/ZrO2 + 6% Y2O3 catalysts rank well below 30% Co/0.2% MgO/Al2O3 catalysts in both activity and selectivity. The catalysts with the smallest average particle size turned out to be the most active.