The reduction of a 25 wt% Co/SiO2 catalyst for the hydrocarbon synthesis has been followed by several techniques : XRD, TPR, XPS and in situ EXAFS. Before reduction the cobalt is present as a Co3O4 spinel phase. A two-step reduction of Co3O4 to CoO and then to Co degrees is observed by EXAFS. This is consistent with XPS (surface) and TPR or XRD (bulk) studies. During CO/H-2 reaction, cobalt is always in the metallic state (EXAFS). The coordination number of cobalt has been determined at each reduction step and during CO hydrogenation reaction.