The structural properties of three Fe-based Fischer-Tropsch synthesis (FTS) catalysts containing different amounts of Cu, K and SiO2 additives were investigated during pretreatment and FTS in a fixed bed-like reactor using combined in situ X-ray absorption fine structure (XAFS)/wide angle X-ray scattering (WARS) techniques. This combination enabled acquisition of complementary information regarding the local environment of iron atoms from XAFS and crystalline phases from WAXS during H-2 and CO/H-2 pretreatment and FTS at 1 bar. The presence of the SiO2 support and promoter elements significantly influenced the structural properties of the catalysts after pretreatment. After FTS, H-2 pretreated catalysts mainly consisted of amorphous theta-Fe3C clusters (unsupported catalysts) or amorphous iron (II) silicate (supported catalyst), while the CO/H-2 pretreated catalysts all consisted mainly of gamma-Fe and chi-Fe5C2. Catalysts activated in CO/H-2 showed superior stability and activity, while H-2 pretreated unsupported catalysts deactivated rapidly during the first 15 h of FTS. (C) 2009 Elsevier Inc. All rights reserved.