We examined the effect of the activation process on the structural and morphological characteristics of a cobalt-based catalyst for Fischer–Tropsch synthesis. A 10wt.% Co/SiO 2 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 773K 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 H 2 /CO activation.