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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 (WAXS) techniques. This combination enabled acquisition of complementary information regarding the local environment of iron atoms from XAFS and crystalline phases from WAXS during H₂ and CO/H₂ 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, H₂ pretreated catalysts mainly consisted of amorphous θ-Fe₃C clusters (unsupported catalysts) or amorphous iron (II) silicate (supported catalyst), while the CO/H₂ pretreated catalysts all consisted mainly of γ-Fe and χ-Fe₅C₂ …