The decomposition of a nanoconfined mixture of lithium–magnesium borohydride, Li11BD4–Mg(11BD4)2, has been investigated and compared to the corresponding mixture in the bulk form. The systems were investigated by thermal analysis, small-angle neutron scattering, 11B nuclear magnetic resonance and transmission electron microscopy. The dehydrogenation temperatures decreased by up to 60 °C in the nanoconfined system, with gas evolution following different steps, compared to the behaviour of the bulk material under the same conditions. Most importantly, desorption from the nanoconfined hydride proceeds without formation of diborane, B2D6, which evolves from the bulk mixture. From small-angle neutron scattering, differences in morphology between the bulk and the nanoconfined systems are also demonstrated. Evidence of a complete decomposition has been found in the nanoconfined system, after heating up to 460 °C. Furthermore, 11B NMR data show that nanoconfinement inhibits the formation of dodecaborane, [B12D12]2−, during decomposition, a result which is important for practical applications of borohydrides.