We examined the effect of the activation process on the structural and morphological characteristics of a cobalt-based catalyst for Fischer-Tropsch synthesis. A 10 Wt-% CO/SiO2 catalyst prepared by wet impregnation was separately activated under H-2, CO or a H-2/CO Mixture. The structural changes during activation from 298 to 773 K were studied by in situ X-ray diffraction. Catalysts were examined by SEM, TEM, XPS and in situ DRIFT-MS. The H-2/CO activation produced redispersion of cobalt particles and simultaneous carbon nanostructures formation. The catalyst showed the highest performance in the Fischer-Tropsch synthesis after the H2/CO activation. (c) 2006 Elsevier B.V. All rights reserved.