Compounds of interest for chemical hydrogen storage at near ambient conditions are specifically tailored to be relatively unstable and thereby desorb H-2 upon heating. Their decomposition must be performed in the absence of impurities to achieve clean dehydrogenation products, which is particularly challenging for an emerging class of microporous complex hydride materials, such as gamma-phase Mg(BH4)(2), which exhibits high surface area and readily adsorbs (sometimes undesired) molecular species. We present a novel strategy toward the purification of yMg(BH4)(2) using supercritical nitrogen drying techniques, (1) showing that clean hydrogen can be released from Mg(BH4)(2) under mild conditions and (2) clarifying the origin of diborane among the decomposition products of stable borohydrides, a topic of critical importance for the reversibility and practical applicability of this class of hydrogen storage compounds. This technique is also widely applicable in the pursuit of the high-purity synthesis of other porous, reactive compounds, an exciting future class of advanced functional materials.