¹²⁹I is a radioactive isotope (T_1/2= 15.7 million years) produced through natural and anthropogenic pathways, but the anthropogenic production is presently dominating the Earth’s surface environments. Sparse data from previous investigations in the Baltic Sea clearly indicated the occurrence of ¹²⁹I at levels 3-4 orders higher than natural pre-atomic era (before 1940) without comprehensive evaluation of distribution and inventory. In this thesis extensive data on the distribution and inventory of iodine isotopes, ¹²⁹I and ¹²⁷I, and their species in waters of the Baltic Sea, Kattegat and Skagerrak are presented and used for estimation of water masses exchange and impact on the environment.  To fulfill these objectives seawater samples were collected in August 2006 and April 2007 in the Baltic Proper, Kattegat and Skagerrak as well as in December 2009 in the Bothnian Sea. After elaborative chemical separation of total iodine and iodine species, the analysis was performed using ICP-MS for ¹²⁷I and AMS for ¹²⁹I. The results reveal considerable differences in ¹²⁹I concentration in terms of spatial and temporal variability and expose relatively high concentrations in the deep waters. Inventory estimates show higher amounts of ¹²⁹I in August 2006 (24.2 ±15.4 kg) than in April 2007 (14.4± 8.3 kg) within the southern and central Baltic Proper, whereas almost a constant inventory is found in the Kattegat Basin. Relatively high ¹²⁷I^-/¹²⁷IO₃^- and ¹²⁹I^-/¹²⁹IO₃^- values in water of the Baltic Proper suggest effective reduction of iodate at a maximum rate of  8×10^-7 (¹²⁷IO₃^-) and 6×10^-14 (¹²⁹IO₃^-) (g/m³.day). The combination of ¹²⁹I and ¹²⁷I as tracers of water circulation in the Baltic Sea suggest that upwelling deep basinal water occurs into the surface along the Gotland deep and intrusion of North Atlantic water into southern Baltic. Furthermore, ¹²⁹I-based model inventory reveals inflow of 330 km³/y (230-450 km³/y) water from the Kattegat into the Baltic Proper. Water exchange between the Baltic Proper and the Bothnian Sea and vice versa is estimated at 980 km³/y (600-1400 km³/y) and 1180 km³/y (780-1600 km³/y) respectively. Finally, an environmental assessment of radioactivity associated with ¹²⁹I burden in the Baltic Sea region is discussed.