An in situ time-resolved XRD/XANES/EXAFS investigation combined with on-line evaluation of catalytic performance indicates considerable structural transformations of the conventional Pt-promoted alumina supported cobalt catalyst under the realistic conditions of Fischer-Tropsch synthesis. Cobalt and platinum are reduced into metallic states after pre-treatment in hydrogen at 623K and form Co-Pt bimetallic nanoparticles. Depending on catalyst and reaction conditions, the metallic nanoparticles can undergo sintering, carbidisation, and fcc-hcp phase transformations. The observed changes in the catalyst structure significantly affect the performance in the Fischer-Tropsch reaction. Cobalt hcp phase seems more active in Fischer-Tropsch synthesis than cobalt fcc metal phase. (C) 2010 Elsevier B. V. All rights reserved.