We herein report on the synthesis of mixed alkali cluster intercalated fullerides NaxLiy-xC60 (y = 12; x = 1-6) by a two-steps mechanochemical reaction of fullerene with sodium and lithium. These compounds crystallize in the cubic lattice of C-60 displaying a contracted lattice parameter with respect to the Na6C60 parent structure. The analysis of the hydrogen sorption behaviour shows a slight decrease in the dehydrogenation enthalpy for y = 12 with respect to the sodium free member. Raman spectroscopy highlighted a partial electron transfer from alkali metals to C-60, suggesting the presence of charged sodium/lithium clusters. Finally, we applied muon spectroscopy to understand the different hydrogenation mechanisms in NaxLi6-xC60 and NaxLi12-xC60 and explain their different performance. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.