The compounds RENi4Mg have been elaborated by mechanical alloying (MA) and a subsequent heat treatment at 600°C for 1 hour. The lattice parameters are in good agreement with those already reported: 7.165 Å for LaNi 4Mg, 7.030 Å for CeNi4Mg and 7.038 Å for GdNi4Mg. The hydrogen sorption behaviours are strongly influenced by the rare earth used: for La compound, hydrogen absorption leads to a structural change and no desorption can be observed, for Ce compound, no reaction towards hydrogen can be noticed and for Gd compound, hydrogen can be absorbed and desorbed reversibly at room temperature. Moreover for GdNi4Mg, no structural change is reported and only a slight volume expansion can be noticed (e.g. 5%). The MA process has also been used successfully for the elaboration of GdNi4-xAlxMg up to x = 1. In these substituted compounds, hydrogen can also been absorbed and desorbed reversibly at room temperature.