Iodine, as potassium iodide in solution, was added to samples of 24 surface soils, 15 subsoils and 16 mixtures of sand with other materials representing soil components, at 10 μg iodine/g soil. The extent of volatilisation of the added iodine was measured after 30 days' exposure in a well-ventilated room. With many of the surface soils volatilisation was negligible although with an acid sandy podsol it amounted to 57 per cent of the iodine added. Eleven of the subsoils induced volatilisation amounting to > 10 per cent of that added. With sand alone, having a pH of 5.7, volatilisation amounted to 100 per cent, and with the mixtures it ranged from nil to 100 per cent. Organic matter reduced volatilisation, probably by retaining the iodine in bound form. Montmorillonite, kaolinite and ferric oxide also reduced volatilisation in comparison with sand alone, but had less effect than did organic matter. Calcium carbonate, although in general reducing volatilisation, probably through its influence on pH and hence on retention by other materials, caused no reduction when added to sand alone.