Аннотация:The transition zone of “Khatanga River – the Laptev sea” was studied by isotope method. The average hydrogen isotope composition of water in the transition zone “river-sea” was determined (δD=-149.5 ‰).Khatanga bay Water is the mixture of sea waters, Khatanga river run-off and river ice melted waters. As the distance from the river the contribution of fresh waters decreases.