In the search for energy storage materials, metal octahydrotriborates, M(B3H8)n, n=1,2, are promising candidates for applications such as stationary hydrogen storage and all solid-state batteries. Therefore, we studied the thermal conversion of unsolvated Mg(B3H8)2 to BH4-: as synthesized, and in the presence of MgH2. The conversion of our unsolvated Mg(B3H8)2 starts at ~100˚C and yields ~22 wt% of BH4- along with the formation of (closo-hydro)borates and volatile boranes. This loss of boron (B) is a sign of poor cyclability of the system. However, the addition of activated MgH2 to unsolvated Mg(B3H8)2 drastically increases the thermal conversion to 85-88wt% of BH4- while simultaneously decreasing the amounts of B-losses. Our results strongly indicate that the presence of activated MgH2 substantially decreases the formation of (closo-hydro)borates and provides the necessary H2 for the B3H8-to-BH4 conversion. This is the first report of a metal octahydrotriborate system to selectively convert to BH4- under moderate conditions of temperature (200 °C) in less than 1h, making the MgB3H8-MgH2 system very promising for energy storage applications.