On the hydrogen embrittlement of Fe-1.0 mol% Cu and Fe-1.5 mol% Cu alloys, the effect of the copper precipitation hardening was studied. The hydrogen diffusivity in the solution treated Fe-Cu alloy is smaller than that in pure iron, and approaches it as the strength increases by aging. On the contrary, the hydrogen concentration in the solution treated Fe-Cu alloy is larger than that in pure iron, and decreases as the strength increases by aging. These imply that the copper, which is a solute atom in iron, acts as a trap site for hydrogen, and this effect decreases when the copper atoms form a cluster. Tensile tests were carried out in the atmosphere and with hydrogen charging electrochemically. The ductility loss of the fracture elongation by hydrogen slightly decreases with the aging time in both alloys. Then, the sensitivity to hydrogen embrittlement of these alloys does not increase, though the strength increases by aging. It is considered that the precipitation of copper reduces the hydrogen concentration and inhibits the formation of the dislocation cellular substructure where the quasi-cleavage fracture is generated. © 2004 Elsevier B.V. All rights reserved.