This work highlights the dehydrogenation mechanisms of a 0.62LiBH4–0.38NaBH4 mixture in the range of 25–650 °C in flowing Ar. The dehydrogenation starts from 287 °C followed by two decomposition steps at 488 °C and 540 °C. These peak temperatures are in the range of 470 °C (for pure LiBH4)–580 °C (for pure NaBH4) due to different Pauling electronegativity values for Li+ (0.98) and Na+ (0.93) that affects the stability and decomposition temperatures. The 1st step of dehydrogenation is accompanied with precipitation of LiH, Li2B12H12 and B in between 287 and 520 °C; whilst the 2nd step of dehydrogenation is mainly accompanied by the precipitation of Na and B when temperature is higher than 520 °C. The total amount of H2 released is 10.8 wt.% that exceeds the estimated amount (8.9 wt.%), indicating less metal dodecaborate (than that for pure LiBH4) is formed during the decomposition.