Two new derivatives of scandium borohydride, MSc(BH4)4, M = Rb, Cs, were prepared via two different synthetic methodologies – mechanochemical and solvent-mediated. The latter led to products free from the commonly present halide contamination, as evidenced by powder X-ray diffraction, FTIR spectroscopy and TGA/DSC/MS. The rubidium derivative crystallizes in an orthorhombic unit cell of the Pbcm space group in the structure which can be derived from ht-CrVO4, while CsSc(BH4)4 adopts a monoclinic (P21/c) unit cell which has monazite (CePO4) as a structural aristotype. Thermal decomposition of the samples obtained using the two methods was compared, evidencing the influence of lithium chloride on the decomposition reactions as well as chemical identity of the decomposition products. Uncontaminated MSc(BH4)4 salts decompose thermally yielding nearly pure hydrogen with the maximum decomposition rate at 230 °C and 235 °C, for M = Rb and Cs, respectively. Among the by-products of the solvent-mediated synthesis, a new cubic crystalline phase of M3ScCl6, M = Rb, Cs, has been detected.