The data on the distribution of uranium and thorium in groundwaters and fluid-impermeable layers of the Mesozoic sediments in the northern regions of the West Siberian basin are presented. The studied waters are diverse in major-ion chemistry and total salinity (from 2 to 5 up to 63,3 g/L). The concentrations of natural radionuclides vary within broad ranges (mg/L): U (4,90·10^-7 - 5,14#183;10^-3), Th (1,90#183;10^-6 - 8,93#183;10^-3). The Th/U ratio varies from 0,07 to 93,1, and its maximal value is characteristic of the waters from the Valanginian aquifers. The Th/U ratio detected for fluid-impermeable rocks is within rather narrow range, 1,82-5,24, 3,56 on average. Against this background, the lowest values Th/U = 0,07-1,71 are characteristic of the rocks of Bazhenov formation, known for its natural radioactivity to be related to uranium. The radionuclide composition of groundwaters of the oil and gas bearing sediments with high Th/U ratios was formed under strongly reductive conditions, when thorium is more mobile than uranium. The evolution of the sedimentary basin led to widespread water expulsion processes and redistribution of U and Th in the water - rock system. Under the oxidative geochemical settings of the upper hydrodynamic zone, U is much better soluble than Th, though it also can be adsorbed on the surface of minerals and get bound into secondary mineral phases. No increase in U and Th concentrations with an increase in the age of water-bearing sediments was detected in the reductive geochemical settings. The results obtained provide for the first time the confirmation of thorium-based nature of radioactivity of the groundwaters in one of the world's largest West Siberian artesian basin.