The mixtures of scandium hydride ScH 2 and metal boride MB n , which is MgB 2 or CaB 6 , were hydrogenated by mechanical milling under hydrogen pressure at room temperature. ScH 2 –MgB 2 and ScH 2 –CaB 6 desorbed 3.4 and 2.3 mass% of H 2 , respectively, with peaks below 300 °C. The results of synchrotron radiation X-ray powder diffraction and X-ray absorption spectroscopy at the Sc K-edge indicated that ScB 2 was produced by milling. Fourier-transform infrared spectroscopy suggested that hydrogen was stored as B–H bonds in the as-milled samples. Nuclear magnetic resonance spectroscopy clarified the presence of metal borohydrides M(BH 4 ) 2 (M = Mg and Ca) in the as-milled ScH 2 –MB n mixtures. These results indicate that M(BH 4 ) 2 is synthesized by milling the ScH 2 –MB n mixtures under hydrogen pressure at room temperature, and hydrogen was desorbed from M(BH 4 ) 2 . The by-products of M(BH 4 ) 2 are MgH 2 in the M = Mg case, which was observed by transmission electron microscopy, and ScB 2 in both cases.