Characteristic features of ultrafiltration water treatment and water treatment using ultrafiltration in combination with complexation for water polluted with U(VI) were investigated. Polyethylenimine (PEI) was used as the complexing agent. The relationship of U(VI) removal as a function of pH and the concentration ratio of C-PEI/C-U(VI) were also studied. It was shown that if no polyelectrolytes were present, the retention coefficient of U(VI) reached 0.91-0.95 at pH 9.0. If polyethylenimine were used in the concentration ratio of C-PEI/C-U(VI) = 4:1, the retention coefficient using a UPM-20 membrane reached 0.999 at the pH range 5-9. The above values were obtained while using branched structure polyethyleneimine with a molecular mass of 60,000. It was shown that the volume flux drops from 24 to 15 mum/s with increasing pH. The addition of the polyelectrolyte insignificantly changed the volume flux.