The hydrogen storage capacity of single-walled and endohedrally Mg doped C-120 composite system has been investigated theoretically by semiempirical quantum chemical treatment at the level of PM3 (RHF) type calculations. The structures are found to, be stable but endothermic in nature. (7H(2)+ Mg)@C-120 structure has the smallest heat of formation value among the series of (nH(2)+Mg)@C-120 molecules considered. Some structural and physicochemical data are also reported. (c) 2005 Elsevier B.V. All rights reserved.