The work reported herein describes the structure and behaviour of alumina-supported nano-sized metallic cobalt particles under realistic activation and working Fischer-Tropsch (FT) synthesis conditions (P-tot = 20 bar). The combination of Quick-XAS for monitoring the change of the local order around Co with time resolved Raman spectroscopy for monitoring hydrocarbons produced by FT synthesis provides a better comprehension of the behaviour of FT catalysts in operation. This is of prime importance to improve cycle length, catalytic activity and selectivity towards heavy paraffins of FT cobalt-based catalysts. Under the activation process used in this work, we evidenced the formation of Co nanoparticles which undergo a reversible oxidation/reduction process depending on the water partial pressure in the cell controlled by the reaction temperature and CO conversion. (C) 2012 Elsevier B.V. All rights reserved.