Abstract Iodide binding by humic acid was studied. The reaction was dose, temperature and pH dependent. A maximum binding of 88% was reached with 2 mg humic acid in 0.15 M Tris buffer. The efficiency of iodide binding by the buffer becomes higher whenever the buffer is slightly alkaline. Iodide binding by humic acid at 60°C is more efficient than that at room temperature or 4°C. The reaction was very rapid and reached equilibrium in 2 h. There are two binding sites for iodide in humic acid. The iodide binding by humic acid is inhibited by depleting of oxygen. The reaction was also inhibited by superoxide dismutase, catalase, thiourea, glutathione and dithiothreitol, but butylated hydroxytoluene and ascorbic acid did not inhibit it. In conclusion: (a) Humic acid can bind iodide efficiently in various buffers, (b) the binding is probably a reaction of oxidation reduction, (c) free radicals are involved in the reaction and (d) free radicals are present in the humic acid.