The hydrogen absorption properties of metal closo-borate/metal hydride composites, M$_2$B$_{10}$H$_{10}$–8MH and M$_2$B$_{12}$H$_{12}$–10MH, M = Li or Na, are studied under high hydrogen pressures to understand the formation mechanism of metal borohydrides. The hydrogen storage properties of the composites have been investigated by in situ synchrotron radiation powder X-ray diffraction at p(H$_2$) = 400 bar and by ex situ hydrogen absorption measurements at p(H$_2$) = 526 to 998 bar. The in

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... experiments reveal the formation of crystalline intermediates before metal borohydrides (MBH$_4$) are formed. On the contrary, the M$_2$B$_{12}$H$_{12}$–10MH (M = Li and Na) systems show no formation of the metal borohydride at T = 400 °C and p(H$_2$) = 537 to 970 bar. 11B MAS NMR of the M$_2$B$_{10}$H$_{10}$–8MH composites reveal that the molar ratio of LiBH4 or NaBH$_4$ and the remaining B species is 1 : 0.63 and 1 : 0.21, respectively. Solution and solid-state 11B NMR spectra reveal new intermediates with a B : H ratio close to 1 : 1. Our results indicate that the M$_2$B$_{10}$H$_{10}$ (M = Li, Na) salts display a higher reactivity towards hydrogen in the presence of metal hydrides compared to the corresponding [B$_{12}$H$_{12}$]$^{2-}$ composites, which represents an important step towards understanding the factors that determine the stability and reversibility of high hydrogen capacity metal borohydrides for hydrogen storage.