The dissociative chemisorption of molecular hydrogen on charged and neutral aluminum clusters Al12X (X = Mg, Al, Si) was investigated using DIFT and a modified G3(MP2)-RAD procedure. Reacdon barriers and enthalpies were determined for both neutral and singly charged clusters. The lowest barrier for dissociative adsorption of H-2 on a neutral cluster was found for the Al12Mg cluster, whereas the highest barrier was found to be on the closed-shell Al12Si. The interaction of H-2 with Al-13(+) is found to proceed via an association complex that. is 0.07 eV lower in energy than the isolated species and from which the barrier to H-2 dissociative adsorption is only 0.16 eV. The most exothermic reaction of H-2 with Al12X occurs for the Al-13(+)/H-2 system. In comparison, reactions with the closed-shell Al-13(-) and Al12Si clusters are found to be endothermic. The barriers for H-2 desorption from the dihydrogenated Clusters are generally quite substantial.