To study the effect of nickel content in iron-nickel alloys on the hydrogen-induced phenomena and susceptibility to hydrogen embrittlement, X-ray diffraction analysis and low strain rate tensile tests were performed for hydrogen charged iron-nickel alloys containing nickel 29-50%. The results obtained are summarized as follows. (1) Hydrogen generated by elecrolytic charging reacts with iron-nickel alloys to form a solid solution and f. c. c. hydride. (2) The tendency for hydride formation increases with increasing nickel content. (3) Electrolytic charging of hydrogen into lower nickel alloys causes the same martensitic transformation as in the cases of cold working and subzero treatment. (4) Hydrogen existing in the form of solid solution suppresses the martensitic transformation in subzero treament. (5) In 33% nickel alloy, no or little hydrogen embrittlement occurs. Alloys containing nickel either below or above 33% embrittled. This embrittlement in lower nickel alloys is due to martensite formation during hydrogen charging, tensile tests and subzero treatment, while in higher nickel alloys it is due to hydride formation.