Iodine-129 is a radionuclide of major concern in the international safety assessments for deep geological storage and disposal of nuclear waste because it migrates quickly through the geosphere to the biosphere and then from the soil to humans through the food-chain. However, in organic soils the 129I may be immobilized over a long time period, and so these soils represent a potential accumulation point in the biosphere. Effects of long residence times of iodine in soils are scarce. The present paper gives some insight on the aging of stable iodine under natural conditions. Stable iodine was introduced as KI in 1987 at the base of a sphagnum bog to simulate arrival of iodine via a groundwater discharge from the geosphere. Fifteen years later, the groundwater, the soil and the vegetation have been sampled and analysed for iodine. The results we present give insight on the mobility of “aged" iodine, the retention properties of the peat, and provide iodine transfer factors for native boreal plant species. This study demonstrates bogs present good sinks for iodine and limit the transfer of iodine to some of the “wildlife" food-chains.