The concentrations of dissolved total iodine and iodate were determined in the water samples collected from various depths at 14 stations located in the upper St. Lawrence Estuary. The concentrations of total iodine and iodate ranged from undetectable (< 0.008 μM) to 0.40 μM and from undetectable to 0.36 μM, respectively, and salinities ranged from 0 to 33.9‰. The concentrations of total iodine and iodate increased with increasing salinity, and they correlated linearly with salinity. This implies that both total iodine and iodate behave conservatively in the estuary. The concentration of iodide, calculated as the difference in the concentrations of total iodine and iodate, was low throughout the estuary. Iodate was the major species of dissolved iodine in the upper St. Lawrence Estuary.