After preparing Mg2Ni1-xFex (x=0.005, 0.01, 0.02 and 0.08) alloys, they were studied by Mossbauer resonance. The Mossbauer spectra of x=0.005 and 0.01 alloys exhibit two doublets (doublet 1, 2). That of x = 0.02 alloys shows two doublets (doublet 1, 2) and one sextuplet, and that of x=0.08 alloy has only one sextuplet. Doublet 1 for x = 0.005, 0.01 and 0.02 alloys is considered to result from a fraction of Fe in excess showing a superparamagnetic behavior. Doublet 2 is considered to result from the Fe substituted for Ni in the Mg,Ni phase. The values of isomer shift 0.25-0.29 mm/s suggest that the iron exists in the state Fe+3. The result that the quadrupolar splitting of doublet 2 is not zero shows that the distribution of electrons around the iron is asymmetric. Their values for the doublet 2, 1.21-1.39 mm/s, approach the value of quadrupolar splitting for the oxidation number +3. The sextuplet showing the magnetic hyperfine interactions results from the iron, which has not substituted the nickel in the Mg2Ni phase. The Mossbauer spectrum of the x = 0.01 alloy hydrided at 723 K shows one doublet and sextuplet. The segregation of Fe is considered to result from the reaction of the Mg2Ni phase with the oxygen contained in the hydrogen as impurity. A part of the segregated iron remains in the form of small particles and gives the doublet of the Mossbauer spectrum. The other part of this iron reacts with the nickel to form an Fe-Ni alloy magnetically ordered at room temperature. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.