Iodine 129 is released by reprocessing plants in northwestern Europe in quantities greatly exceeding its natural inventory and other anthropogenic sources. Data are presented from nine profiles of 129I in the Greenland and Norwegian Seas and northern North Atlantic Ocean, collected during the Intergovernmental Oceanographic Commission's Baseline Survey of Contaminants in the North Atlantic Ocean. Anthropogenic 129I is evident throughout the data set. The distribution of 129I in the major water masses sampled provides an excellent illustration of the unique source function of 129I, which results in stronger labeling of the northern-source overflow waters and the Deep Western Boundary Current of the North Atlantic Ocean than for many other tracers. Box models are used to explore the behavior and transport of 129I. These preliminary model simulations agree well with available measurements, pointing the way to future quantitative applications of this tracer. ¿ 2001 American Geophysical Union