In order to analyze the influence of the crystal size of the active phase in the Fischer-Tropsch synthesis (FTS), two iron catalysts, supported on MCM-41 and SBA-15, were prepared by incipient wetness impregnation and tested in this reaction. The solids were characterized by atomic absorption spectroscopy, N-2 adsorption, X-ray diffraction at low angles and Mossbauer spectroscopy. The activated catalysts showed the same iron species. However, the average crystal size of the active phase is about three times higher in SBA-15 than in MCM-41. Besides, both systems displayed a narrow size distribution. The catalyst with higher crystal size (Fe/SBA-15) presented higher conversion, higher chain growth and lower CH4 production. These results would indicate that the FTS is a "structure sensitive" reaction in a sizes range within, approximately, 3-10 nm. (C) 2010 Elsevier B.V. All rights reserved.