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We investigated changes in iodine (¹²⁹I) solubility and speciation in nine soils with contrasting properties (pH, Fe/Mn oxides, organic carbon and iodine contents), incubated for nine months at 10 and 20°C. The rate of ¹²⁹I sorption was greater in soils with large organic carbon contents (%SOC), low pH and at higher temperatures. Loss of iodide (I⁻) from solution was extremely rapid, apparently reaching completion over minutes–hours; iodate (IO₃^-) loss from solution was slower, typically occurring over hours–days. In all soils an apparently instantaneous sorption reaction was followed by a slower sorption process for IO₃^-. For iodide a faster overall reaction meant that discrimination between the two processes was less clear. Instantaneous sorption of IO₃^- was greater in soils with high Fe/Mn oxide content, low pH and low SOC content, whereas the rate of time-dependent sorption was greatest in soils with higher …