The role of different functional groups in humic acid was investigated for complex formation during the interaction of Fe +2 and Fe +3 ions with humic acid, oxidized humic acid, acetylated humic acid and decarboxylated humic acid. The formation of two types of complexes, ionic and non-ionic, was observed. The results indicate that the proton releasing groups, e.g. carboxyl and phenolic hydroxyl groups, are responsible for the formation of a ionic complex, and the formation of a non-ionic complex involves the carboxyl and carbonyl (keto) groups. The percentage of the ionic iron in the complex increases and that of the non-ionic iron decreases with increase of pH.