Iodine speciation plays a significant role in iodine volatilizing into atmosphere from the seas, as well as serving as a biological indicator. Despite this importance, the data on iodine species revealed inconclusive evidence of what factors controlling speciation transformation. We here present new data on profiles of (129)I speciation in the Baltic Proper during November 2009. Along with the two earlier investigations (August 2006 and April 2007), an assessment of seasonal variation of (129)I species is presented. The results show that, due to the anoxic nature of Baltic Proper, presence of (129)IO(3) (-) in the Baltic Proper does not follow an obvious seasonal cycle, as the case with (129)I(-). Concentrations of (129)I(-) in the Baltic Proper exhibit higher values in summer than the other two seasons (spring and winter), which might be associated with degrading of organic matter and release from sediment to water column that is more pronounced during summer. (129)I(-) in surface water from the three seasons does not reflect the release function from the reprocessing facilities during the period April 2007 to November 2009. Consequently variability of (129)I(-) in surface seawater of the Baltic Proper depends, to some extent, on local physical as well as biochemical conditions.