As a consequence of an accidental fusion of a Cs-137 source in a steel production factory, 40 m(3) of radioactive liquids were generated in the plant decontamination process. This radioactive liquid waste was treated by a reverse osmosis plant, managing to declassify more than 36 m(3). The remaining 4 m(3) were treated by evaporation reducing the final volume to less than 1 m(3). This paper describes the reverse osmosis plant used and the process of treatment, and presents the results of reverse osmosis membrane performance in the course of the treatment.