Thermal decomposition of magnesium borohydride, Mg(BH4)2, in the solid state was studied by a combination of PCT, TGA/MS and NMR spectroscopy. Dehydrogenation of Mg(BH4)2 at 200 °C en vacuo results in the highly selective formation of magnesium triborane, Mg(B3H8)2. This process is reversible at 250 °C under 120 atm H2. Dehydrogenation at higher temperature, >300 °C under a constant argon flow of 1 atm, produces a complex mixture of polyborane species. A borohydride condensation mechanism involving metal hydride formation is proposed.