Uranium and thorium quantity in soils and plants of pine forests on the south of Eastern Siberia was investigated. This region is a major wood supplier. The survey of radioactive elements concentration in wood is a subject of practical interest. Concentration of uranium and thorium was determined in soil samples, in trees aboveground parts and roots (Scots pine and Siberian larch) and in ground plants of applying modified methods represented in the manual. Samples analysis included ashing, dissolving in nitric acid, extracting by tributylphosphate, ion-exchange separation and measuring radioactivity on liquid scintillation counter «Rack-Beta».

The major regularity typical for every studied phytogeocoenosis was that the highest thorium and uranium concentration in the ash of various parts of the forest forming species (pine and larch) was reached in the roots; the lowest concentration was in needles. Thorium concentration in the ash from the trees growing on the soddy-podzol soils was noticeably higher than from the ones growing on the soddy-forest soils; however, in terms of this index, the uranium content differed to a less degree.

The average uranium content in the soils of the studied region ranged from 1.44 to 2.23 mg kg^-1, in the wood ash from 0.050 to 0.170 mg kg^-1; the average thorium content reached 4.18 – 6.41 mg kg^-1 in the soils and 0.135 – 0.752 mg kg^-1 in the wood ash.