The complexation affinity of the Aldrich Chemicals humic acid with Co-60 was examined with respect to molecular size of humic acid. Correlations between the affinity and the structures of humic acid were studied. At low humic acid concentration range, Co-60 was interacted with the humic acid of size fraction over 100k Da (HA(100 <)). With increasing humic acid concentration, the Co-60 was preferentially interacted with the 30k-100k Da of humic acid (HA(30-100)). Fractionated HA(100 <) and HA(30-100) were characterized from their FTIR (Fourier Transform Infra-Red), C-13 NMR spectral analyses and acid-base titration curves. The HA(10 <) having aliphatic branched structure, smaller amount of COOH group and large proton exchangeable capacity, seem to show low covalent bonding nature and high ion exchangeability in the complexation. In addition, steric hindrance may affect on the complexation, by winding up like random coils from the branched structure. The HA(30-100) is dominated with the aromatic COOH group and OH group and it may preferentially coordinate to Co-60 by covalent binding. These presumptions were supported by XPS analysis, in which the biding energy of cobalt-humates was discussed. (C) 2011 Elsevier B.V. All rights reserved.