Abstract: By means of x-ray diffraction analysis and Mossbauer spectroscopy study it has been confirmed that the catalytically active structure of the B₁₀₄ CO shift catalyst containing about 7-9% Cr consists of non-stoichiometric magn etite and iron-chromium solid solution. It appears that the iron ions in the B site of the magnetite play more important role than those in the A site, and that the electrons being exchanged rapidly between Fe²⁺ and Fe³⁺ in the B site may take part in the catalytic process.