The differentiation of the isotopic composition of glacial water in some Hornsund glaciers was found to depend on its position with respect to the firn line, situation in the circulation system and on seasonal variations of air temperature. Intensive ice thawing in the ablation zone is marked by decreased values of the tritium content in the discharge of the glacier, whereas water from thawing snow nad firn increases these values. The isotopic differentiation which occures in waters in the main flow systems indicates the magnitude of the shares of water from thawed ice firn and snow in the total drainage of the glacier.