A method has been developed for speciation analysis of ultra low level I-129 in soil using sequential extraction combined with coprecipitation for separation of carrier free iodine and AMS measurement of I-129. Two loess profiles collected from northwest China were analyzed for species of I-129 and I-127. Similar partitioning of I-129 and I-122 was observed in the loess profiles, the distribution of iodine isotopes followed an order of organic > leachable > reducible > residue. The I-129 concentrations and I-129/I-127 ratios decreased exponentially with the depth, and 2 orders of magnitude lower in the deepest layer (60 and 90 cm) compared with the top layer, indicating a significant contribution of anthropogenic input in the upper layer, and high retention of I-129 in soil. The mobility of I-129 in different fractions decreased in an order of leachable > organic > oxides > residue. The results suggest that migration of iodine downwards in the soil profile is a slow process; the oxides and residue are the less mobile fractions of iodine.