Ammonium borohydride, NH₄BH₄, has a high hydrogen content of ρ_m = 24.5 wt% H₂ and releases 18 wt% H₂ below T = 160 °C. However, the half-life of bulk NH₄BH₄ at ambient temperatures and pressures, 6 h, is insufficient for practical applications. The decomposition of NH₄BH₄ (ABH₂) was studied at variable hydrogen and argon back pressures to investigate possible pressure mediated stabilization effects. The hydrogen release rate from solid ABH₂ at ambient temperatures is reduced by 16% upon increasing the hydrogen back pressure from 5 to 54 bar. Similar results were obtained using argon pressure and the observed stabilization may be explained by a positive volume of activation, ca. 73 ± 17 cc mol⁻¹, in the transition state leading to hydrogen release. Nanoconfinement in mesoporous silica, MCM-41, was investigated as alternative means to stabilize NH₄BH₄. However, other factors appear to significantly destabilize NH₄BH₄ and it rapidly decomposes at ambient temperatures into [(NH₃)₂BH₂][BH₄] (DADB) in accordance with the bulk reaction scheme. The hydrogen desorption kinetics from nanoconfined [(NH₃)₂BH₂][BH₄] is moderately enhanced as evidenced by a reduction in the DSC decomposition peak temperature of ΔT = −13 °C as compared to the bulk material. Finally, we note a surprising result, storage of DADB at temperature <−30 °C transformed, reversibly, the [(NH₃)₂BH₂][BH₄] into a new low temperature polymorph as revealed by both XRD and solid state MAS ¹¹B MAS NMR.