The purpose of this study is to compare the thermal and structural stability of single phase Li2B12H12 with the decomposition process of LiBH4. We have utilized differential thermal analysis/thermogravimetry (DTA/TGA) and temperature programmed desorption-mass spectroscopy (TPD-MS) in combination with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy to study the decomposition products of both LiBH4 and Li2B12H12 up to 600 degrees C, under both vacuum and hydrogen (H-2) backpressure. We have synthesized highly pure single phase crystalline anhydrous Li2B12H12 (Pa-3 structure type) and studied its sensitivity to water and the process of deliquescence. Under either vacuum or H2 backpressure, after 250 degrees C, anhydrous Li2B12H12 begins to decompose to a substoichiometric Li2B12H12-x composition, which displays a very broad diffraction halo in the d-spacing range 5.85-7.00 angstrom, dependent on the amount of H released. Aging Pa-3 Li2B12H12 under 450 degrees C/125 bar H-2 pressure for 24 h produces a previously unobserved well-crystallized beta-Li2B12H12 polymorph, and a nanocrystalline gamma-Li2B12H12 polymorph. The isothermal release of hydrogen pressure from LiBH4 along the plateau and above the melting point (T-m = 280 degrees C) initially results in the formation of LiH and gamma-Li2B12H12. The gamma-Li2B12H12 polymorph then decomposes to a substoichiometric Li2B12H12-x composition. The Pa-3 Li2B12H12 phase is not observed during LiBH4 decomposition. Decomposition of LiBH4 under vacuum to 600 degrees C produces LiH and amorphous B with some Li dissolved within it. The lack of an obvious B-Li-B or B-H-B bridging band in the FTIR data for Li2B12H12-x suggests the H poor B12H12-x pseudo-icosahedra remain isolated and are not polymerized. Li2B12H12-x is persistent to at least 600 degrees C under vacuum, with no LiH formation observable and only a ca. d = 7.00 angstrom halo remaining. By 650 degrees C, Li2B12H12-x is finally decomposed, and amorphous B can be observed, with no LiH reflections. Further studies are required to clarify the structural symmetry of the beta- and gamma-Li2B12H12 polymorphs and substoichiometric Li2B12H12-x.