The oxidation of the non-stoichiometric iron oxide Fe1-yO and mixed iron manganese oxides (Fe1-xMnx)(1-y)O (x less than or equal to 0.3) by water forms molecular hydrogen. The course of this reaction was studied in a tubular fixed bed reactor at various temperatures by means of on-line mass spectrometry of the gas phase and by structural investigations of the solid products with powder X-ray diffraction and scanning electron microscopy.