Several commercial ammonia synthesis catalysts have been studied by TPR and XRD in an Fe3O4 and prereduced-passivated state. Extension of TPR experiments up to 1200 degrees C provided information which was yet unknown. From TPR profiles and XRD patterns of Fe3O4 catalysts, some conclusions can be drawn about the catalyst preparation procedure and expected activity. Concerning the prereduced-passivated catalysts, TPR and XRD experiments demonstrate the presence of residual magnetite in some commercial samples, while others were found to be fully prereduced. A quantitative evaluation of the content of residual magnetite was performed. The practical consequences of this situation on catalyst performance in industrial plants are also discussed. The low reproducibility of the passivation degree found in commercial prereduced catalysts, is most probably due to insufficient control of the operating conditions during the passivation step, Finally, the importance of TPR for the characterization of ammonia synthesis catalysts is stressed.