The fact that a magnetic hysteresis loop is observed only in polymerized C-60 and is absent in both pristine and depolymerized C-60 establishes a close link between the weak ferromagnetism and the underlying structure. Furthermore, the correlation between the weak ferromagnetism and hydrogen presence has been explored by H-1 NMR and H-1-C-13 cross-polarization NMR, whereby strong bonding between the H adducts and C-60 has been verified. The adducted hydrogens most likely form bonds with the carbon atoms of inter-C-60 four-membered rings. This conclusion is further supported by the formation energy of adducted H and the exchange energy among adducted H obtained from first-principle calculations on Rh-C-60. (C) 2009 Elsevier B. V. All rights reserved.