Deuterium−hydrogen exchange in solid α-Mg(BH4)2 is demonstrated. Compared to the previously reported exchange reactions in the alkali borohydrides, the temperature at which isotope exchange starts to take place is significantly lower (132 °C vs 200 °C for LiBH4). The activation energy for the deuterium−hydrogen exchange reaction is estimated to be 51 ± 15 kJ/mol. Almost complete isotope exchange was observed by treating solid Mg(BH4)2 for 72 h at 172 °C with 42 bar of D2. Preliminary experiments indicate that under these conditions Ca(BH4)2 also undergoes isotope exchange.