SiO2 covered with MgO has been used as support of iron catalysts in the Fischer-Tropsch reaction. Catalysts of 5% (w/w) iron concentration and 2, 4, and 8% (w/w) of MgO on SiO2 were prepared. Selective chemisorption of CO, volumetric oxidation, and M8ssbauer spectroscopy were used to characterize the type of iron species and the metallic crystal sizes. MgO covers the SiO2 surface and modifies the metallic crystal size. The activity to total hydrocarbons increases with the amount of MgO added. An optimal concentration of about 4% (w/w) was found to have the highest selectivity to olefins.