We herein report on the synthesis of mixed alkali cluster intercalated fullerides Na x Li y−x C 60 ( 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 Na 6 C 60 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 Na x Li 6 −x C 60 and Na x Li 12 −x C 60 and explain their different performance. [ABSTRACT FROM AUTHOR]