In order to study if the SMSI effect produces benefitial results on the Fischer-Tropsch (FT) reaction using the Fe/Nb2O5 system we prepared two catalysts by reduction at two different temperatures. The solids were characterized by Mossbauer spectroscopy, XRD, XPS, N-2 adsorption, atomic absorption. CO chemisorption and volumetric oxidation. It was demonstrated that the support is partially reduced leading to a covering of the Fe degrees crystals by NbOx species. At the higher reduction temperature studied (773 K) this effect produced a very important activity decrease, an increase in methane production and a shift of the product distribution towards to higher molecular weight hydrocarbons. (c) 2007 Elsevier B.V. All rights reserved.