The present study was undertaken to investigate reduction and catalytic behavior of silica and alumina-supported iron Fischer-Tropsch (F-T) catalysts promoted with potassium and copper. It was found that the alumina (Vista Catapal 13) inhibits reduction of iron, whereas the reduction behavior of the silica (Davison Grade 952) supported catalyst was similar to that of the two precipitated iron catalysts synthesized in our laboratory. Supported and precipitated (Fe/Cu/K/SiO2) catalysts were tested in a stirred tank slurry reactor. Initial activity of the silica-supported catalyst, measured by a value of an apparent first-order reaction rate constant, was 20-40% higher than that of the precipitated iron catalysts, whereas the activity of the alumina-supported catalyst was about 50% less than that of the silica-supported catalyst. Silica-supported catalyst had higher gaseous selectivity and lower olefin content than the alumina-supported catalyst and precipitated catalysts. These results were explained in terms of interactions between potassium with supports, resulting in reduction of the potassium promotion effectiveness.