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Summary High‐resolution transmission electron microscopy characterization of molybdenum oxide catalyst precursors is carried out for a better understanding of the mechanisms leading to the active catalytic material. The precipitated material consists of both large‐unit cell crystals and randomly orientated clusters on a 3–5 nm scale. The crystalline clusters are embedded in non‐crystalline material, preventing the formation of large well‐defined inactive orthorhombic MoO3 crystals during activation of the catalyst material.