A powder of an alloy of Ni and Mg, La, Be or Li, consisting of crystallites having a grain size lower than 100 nm and a crystalline structure allowing hydrogen absorption. This powder which is preferably obtained by mechanical grinding, may consist of cristallites of Mg.sub.2 Ni, LaNi.sub.5 or of Ni-based alloys of Be or Li having a grain size lower than 100 nm. The powder may also consist of cristallites of formula Mg.sub.2-x Ni.sub.1+x, x ranging from -0.3 to +0.3, which have a grain size lower than 100 nm, and preferably lower than 30 nm. This crystalline powder is particularly useful for storing and transporting hydrogen. Indeed, it has been discovered that such Ni-based nanocrystalline powder requires no or only one single activation treatment at low temperature to absorb hydrogen. It has also been discovered that the kinetic of absorption and diffusion of hydrogen within the powder is much faster. This can be explained by the presence of a large number of grain boundaries.