The subject of the invention is a composition comprising: a) a hydrogen-fixing alloy of formula ABx where: A is an element chosen from the group comprising La, Ce, Nd, Pr and Mg, or a mixture of these; B is an element chosen from the group comprising Ni, Mn, Fe, Al, Co, Cu, Zr, Sn, or a mixture of these; x is from 3 to 6; b) a magnesium compound in such a proportion that its mass is greater than 0.1% and less than or equal to 5% of the mass of the hydrogen-fixing alloy. This composition can be used as an active material of a negative electrode of a nickel-metal hydride accumulator. Such an accumulator displays a reduced self-discharge. The invention also describes the method of producing such an accumulator.