Alkali-cluster intercalated fullerides (LixC60, NaxC60) proved to be suitable materials for solid state hydrogen storage. In these compounds the hydrogen is chemically absorbed on the fullerene molecule while the intercalated cluster is able to dissociate the hydrogen molecule at moderate temperature and pressure. Here we intend to study the hydrogen binding mechanisms in the new class of mixed sodium and lithium fullerides (Na_{x}Li_{6-x}C_{60}). Thanks to the capability of muons to form a light isotope of hydrogen (muonium, Mu), we will be able to investigate the chemical interactions of Mu with our systems, comparing the results with those obtained with the pure Li and Na intercalated fullerene.