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The mechanisms of bonding of Mn2+, Cu2+, and VO2+ in humic acid extracted from a mineral soil were investigated by electron spin resonance spectroscopy. The Mn(H2O)62+ ion appeared to retain its inner hydration sphere upon adsorption, and was adsorbed at the organic surface by electrostatic attraction. Both Cu2+ and VO2+ entered into rigid surface complexes with oxygen ligands of organic functional groups, however, losing rotational mobility. The different bonding mechanisms of these transition metal ions are explained in terms of ligand field effects. No evidence of highly covalent metal-organic bonds was found.