The paper addresses the effects of silica and carbon supports on the structure and catalytic performance of iron catalysts for high temperature Fischer-Tropsch synthesis. Iron phase composition and dispersion in both calcined and activated catalysts were strongly affected by the support. Hematite was the major iron phase in calcined silica supported catalysts, while carbon supported counterparts contain magnetite. Catalyst activation in carbon monoxide leads to carbidisation of iron oxides to principally Haag iron carbide. Higher Fischer-Tropsch reaction rates were observed on carbon supported iron catalysts compared to silica supported counterparts. The catalytic performance principally depended on iron phase composition rather than on iron dispersion. Iron catalysts supported on carbon nanotubes and activated carbon showed highest activity in Fischer-Tropsch, which could be attributed to the formation of composites of iron carbide and residual magnetite.