Microscopic and microprobe studies of significantly active, fused model iron catalysts with increased amounts of wustite are reported. The catalysts were examined in untreated form and after annealing. Two types of magnetite (isometric and lamellar) were found. A definite influence of the annealing on the relative content of both magnetites was observed. In the annealed sample a third kind of magnetite (enriched in iron) was found. The creation of iron nuclei in the catalyst precursor is discussed and an suitable model is proposed.