Humic acids are iodinated by elemental iodine and, if the iodine is present as iodide, by peroxidase-mediated reactions. It is demonstrated that iodination of humic acids leads to a product with a uniform distribution of iodine. It could not be unambiguously verified whether the enzymatically mediated iodination is a direct reaction between a peroxidase-iodine complex and the humic acid molecule or a two-step reaction in which the enzyme creates elemental iodine, which consecutively reacts with the humic acid. Based on a simple model of a reaction between sites in the humic acids available for iodination and the electrophilic iodinating species, it was concluded that the reaction should be described as an equilibrium with a logarithmic equilibrium constant of approximately 4. The number of sites available for iodination was, in the humic acids studied, determined to be approximately 4×10−4 per gram humic acid. The different parameters influencing the enzymatically controlled iodination of humic acids are discussed.